Is the acquisition of basic-colour terms in young children constrained? For more than a hundred y... more Is the acquisition of basic-colour terms in young children constrained? For more than a hundred years there has been substantial interest in the developmental difficulties young children experience when learning colour terms (Baldwin 1893; Darwin 1877, cited in Bornstein 1985; Nagel 1906). Initially, children may name many different colours with only one colour term and, even when they have acquired several colour terms, their naming of colour sensations is often inappropriate (eg Cruse 1977; Modreski and Goss 1969). Reliable colour naming has previously been reported to appear surprisingly late, at around 4^7 years (eg Bornstein 1985; Heider 1971; Johnson 1977; Mervis et al 1975), and may only be learned through a prolonged period of explicit instruction (Rice 1980). In contrast, the process by which young children learn the names for everyday objects appears effortless and often occurs within a single episode of hearing the word spoken in context (eg Carey 1978). The marked disparity between children's naming of everyday objects and colours suggests that the systematic process of fast mapping between novel labels and their referents that characterises early object word learning (eg Markman 1989
Shape processing involves a progression from local to global analysis. A key aspect of this is th... more Shape processing involves a progression from local to global analysis. A key aspect of this is the binding of distributed local features into an overall form followed by the extraction of the shape independently of its local contrast and spatial scales, so enabling the shape to be encoded based on its proportions without reference to its exact size or retinal location. Here we use contour-and surface-defined radial frequency (RF) patterns in a shape discrimination task, previously thought to reflect a global processing stage that has reached contrast and scale invariance. We compare performance across different spatial scales for a wide range of RF patterns (contour spatial frequencies of 0.7-10.0 cpd, pattern radii of 0.5-10.5°), and sharp-and smooth-edged surface-RF patterns, all at low contrast (5Â detection threshold). We show that shape discrimination thresholds for RF patterns have a complex series of dependencies on stimulus size (radius), contour spatial frequency (thickness) and contrast, with no scale invariance. Our results are at odds with earlier work showing no effect of radius and spatial frequency on discrimination thresholds. We show that this discrepancy can be accounted for by a differential effect of contrast on shape discrimination, with shape invariance only stabilizing at higher contrasts (10-20Â threshold).
Slant is the degree to which a surface recedes or slopes away from the observer about the horizon... more Slant is the degree to which a surface recedes or slopes away from the observer about the horizontal axis. The perception of surface slant may be derived from static monocular cues, including linear perspective and foreshortening, applied to single shapes or to multi-element textures. It is still unclear the extent to which color vision can use these cues to determine slant in the absence of achromatic contrast. Although previous demonstrations have shown that some pictures and images may lose their depth when presented at isoluminance, this has not been tested systematically using stimuli within the spatio-temporal passband of color vision. Here we test whether the foreshortening cue from surface compression (change in the ratio of width to length) can induce slant perception for single shapes for both color and luminance vision. We use radial frequency patterns with narrowband spatio-temporal properties. In the first experiment, both a manual task (lever rotation) and a visual task (line rotation) are used as metrics to measure the perception of slant for achromatic, red-green isoluminant and Scone isolating stimuli. In the second experiment, we measure slant discrimination thresholds as a function of depicted slant in a 2AFC paradigm and find similar thresholds for chromatic and achromatic stimuli. We conclude that both color and luminance vision can use the foreshortening of a single surface to perceive slant, with performances similar to those obtained using other strong cues for slant, such as texture. This has implications for the role of color in monocular 3D vision, and the cortical organization used in 3D object perception.
Purpose: In this study we examine neuroretinal function in five amblyopes, who had been shown in ... more Purpose: In this study we examine neuroretinal function in five amblyopes, who had been shown in previous functional MRI (fMRI) studies to have compromised function of the lateral geniculate nucleus (LGN), to determine if the fMRI deficit in amblyopia may have its origin at the retinal level. Methods: We used slow flash multifocal ERG (mfERG) and compared averaged five ring responses of the amblyopic and fellow eyes across a 35 deg field. Central responses were also assessed over a field which was about 6.3 deg in diameter. We measured central retinal thickness using optical coherence tomography. Central fields were measured using the MP1-Microperimeter which also assesses ocular fixation during perimetry. MfERG data were compared with fMRI results from a previous study. Results: Amblyopic eyes had reduced response density amplitudes (first major negative to first positive (N1-P1) responses) for the central and paracentral retina (up to 18 deg diameter) but not for the mid-periphery (from 18 to 35 deg). Retinal thickness was within normal limits for all eyes, and not different between amblyopic and fellow eyes. Fixation was maintained within the central 4°more than 80% of the time by four of the five participants; fixation assessed using bivariate contour ellipse areas (BCEA) gave rankings similar to those of the MP-1 system. There was no significant relationship between BCEA and mfERG response for either amblyopic or fellow eye. There was no significant relationship between the central mfERG eye response difference and the selective blood oxygen level dependent (BOLD) LGN eye response difference previously seen in these participants. Conclusions: Retinal responses in amblyopes can be reduced within the central field without an obvious anatomical basis. Additionally, this retinal deficit may not be the reason why the LGN BOLD (blood oxygen level dependent) responses are reduced for amblyopic eye stimulation.
1. A method ofproducing red-green and blue-yellow sinusoidal chromatic gratings is used which per... more 1. A method ofproducing red-green and blue-yellow sinusoidal chromatic gratings is used which permits the correction of all chromatic aberrations. 2. A quantitative criterion is adopted to choose the intensity match of the two colours in the stimulus: this is the intensity ratio at which contrast sensitivity for the chromatic grating differs most from the contrast sensitivity for a monochromatic luminance grating. Results show that this intensity match varies with spatial frequency and does not necessarily correspond to a luminance match between the colours. 3. Contrast sensitivities to the chromatic gratings at the criterion intensity match are measured as a function of spatial frequency, using field sizes ranging from 2 to 23 deg. Both blue-yellow and red-green contrast sensitivity functions have similar low-pass characteristics, with no low-frequency attenuation even at low frequencies below 041 cycles/deg. These functions indicate that the limiting acuities based on red-green and blue-yellow colour discriminations are similar at 11 or 12 cycles/deg. 4. Comparisons between contrast sensitivity functions for the chromatic and monochromatic gratings are made at the same mean luminances. Results show that, at low spatial frequencies below 0 5 cycles/deg, contrast sensitivity is greater to the chromatic gratings, consisting of two monochromatic gratings added in antiphase, than to either monochromatic grating alone. Above 0-5 cycles/deg, contrast sensitivity is greater to monochromatic than to chromatic gratings.
ABSTRACT Cross-orientation masking occurs when the detection of a test grating is masked by a sup... more ABSTRACT Cross-orientation masking occurs when the detection of a test grating is masked by a superimposed grating at an orthogonal orientation, and is widely thought to indicate the presence of "cross-channel" interactions mediating contrast normalization. In achromatic vision, modeling and psychophysical experiments have suggested that there are at least two routes to cross-orientation suppression prior to binocular summation: a within-eye (monocular) pathway that is non-adaptable, and an interocular (dichoptic) adaptable pathway that is cortical and mediates mutual suppression between the eyes (Baker et al., Neuroscience, 146, 2007: Meese & Hess, 2004). Here we develop this two-stage model and test its application to color vision. Test and mask stimuli were red-green isoluminant Gabors presented orthogonally. TvC masking functions were obtained for three spatial frequencies (0.375, 0.75 & 1.5cpd at 2Hz) for monocular, binocular and dichoptic presentations in four subjects. We generalized the two-stage model so that it could be used to fit the monocular, dichoptic and binocular TvC functions. We determined the weight of suppression parameters for the monocular and dichoptic suppression, with the remaining parameters fixed on the basis of known values. We find that the two-stage model is a good fit to the chromatic data, supporting the idea that color and achromatic vision use the same two routes to cross-orientation suppression. The dichoptic pathway shows greater suppression than monocular in color vision. It is also unselective as it can be activated by achromatic or chromatic contrast (Mullen et al., 2012). We find no effect of spatial frequency on either pathway.
The apparent contrast of a plaid is a reflection of the neural relationship between the responses... more The apparent contrast of a plaid is a reflection of the neural relationship between the responses to its two orthogonal component gratings. To investigate the perceived contrast summation of the responses to component gratings in plaids, we compared the apparent contrasts of monocular plaids to a component grating presented alone across chromaticity and spatial frequency. Observers performed a contrast-matching task for red-green color and luminance stimuli at low-and medium-spatial frequencies. Using the measured points of subjective equality between plaids and gratings, we evaluate perceived contrast summation across conditions, which may vary between 1 (no summation) and 2 (full summation). We show that achromatic plaids have higher perceived contrast summation than chromatic plaids. The greatest difference occurs at the medium-spatial frequency, with summation highest for achromatic plaids (1.87) and lowest for chromatic plaids (1.49), while at low-spatial frequencies, there is a smaller summation difference between achromatic (1.72) and chromatic (1.65) plaids. These results are consistent with recent theories of distinct cross-orientation suppression and summation mechanisms in color and luminance vision. Two control experiments for binocular versus monocular viewing, and the overall size of the stimulus patches did not reveal any differences from our main results.
We measure the orientation tuning of red-green colour and luminance vision at low (0.375 c/deg) a... more We measure the orientation tuning of red-green colour and luminance vision at low (0.375 c/deg) and mid (1.5 c/deg) spatial frequencies using the low-contrast psychophysical method of subthreshold summation. Orientation bandwidths of the underlying neural detectors are found using a model involving Minkowski summation of the rectified outputs of a bank of oriented filters. At 1.5 c/deg, we find orientation-tuned detectors with similar bandwidths for chromatic and achromatic contrast. At 0.375 c/deg, orientation tuning is preserved with no change in bandwidth for achromatic stimuli, however, for chromatic stimuli orientation tuning becomes extremely broad, compatible with detection by non-oriented colour detectors. A non-oriented colour detector, previously reported in single cells in primate V1 but not psychophysically in humans, can transmit crucial information about the color of larger areas or surfaces whereas orientation-tuned detectors are required to detect the colour or luminance edges that delineate an object's shape.
In our visual world we can distinguish with ease between chromatic and luminance contrasts. Howev... more In our visual world we can distinguish with ease between chromatic and luminance contrasts. However, in our retinae most neurones are responsive to both chromatic and luminance changes and therefore send ambiguous or 'multiplexed' information to the higher visual centres. Psychophysical evidence suggests that some cortical process must subsequently separate out this information into its chromatic and luminance components. The purpose of this communication is to review and critically evaluate the different existing schemes for doing this. To assist in this evaluation a linear systems analysis is employed in which model cortical neurones are imputed with the property of providing information about either colour or luminance. It is concluded that there is currently no unified scheme available to explain a separation of colour and luminance information in the visual system. Some theoretical considerations and most promising approaches to solving the problem are noted, but it is suggested that there may be definite limits to the ability of the visual system to achieve complete separation of colour and luminance from the retinal signal.
We have investigated the spatial transfer characteristics of the mechanisms sensitive to color in... more We have investigated the spatial transfer characteristics of the mechanisms sensitive to color in the human visual system using a method of shnultaneous spatial masking with isol~uaut chromatic sthuuli. The test sthnuli were Gaussian enveloped &-green gratings of three spatial frequencies in the lowpasu region of the color domain (0.25, 0.5 and 1 c/deg). The maskhrg stimuli were redgreen gratings at the orientation and phase of the test, presented at the same spatial frequency, and at & 1, and + 2 octaves from its spatial frequency. We obtained test contrast &e&old as a function of mask contrast for a wide range of mask contrasts (TvC functions). Tuuhrg fuuctiorw were derived from linear fits of the masking data, by taking the mask contrast that doubled the mhdmum test thre&old at each spatial frequency. Chromatic tuning functions show handpass characteristics for all teat spatial frequencies examined with an average full bandwidth at half-height of 2.6 octaves, which is shnilar to the huninance bandwidths obtained under comparable conditions. Thus, our results suggest that the color contrast sensitivity function is the upper envelope of a range of bandpass mechanisms whose peaks extend to very low spatial frequencies.
In this study, we consider how colour contrast can be used to integrate form and how it interacts... more In this study, we consider how colour contrast can be used to integrate form and how it interacts with luminance contrast in the task. The performance of form integration was assessed by measuring the detection of a winding "contour" of aligned gabor elements embedded in a background of randomly oriented gabors, using both luminance and isoluminant (red/green) contrast. Performance on the task improves with gabor element contrast, and identical performance for colour and luminance contour detection is achieved at high screen contrasts, showing that coiour is able to support a complex form integration task. In a second experiment, we investigate whether colour and luminance contrast can be combined in contour integration by measuring the detection of a path with alternating isoluminant colour and luminance elements. We find that contour detection uses both colour and luminance information cooperatively, but performance is much poorer than would be expected from a single common contour integration process which fails to distinguish the two types of contrast. This suggests that there are specific contour integration processes for colour and luminance. In a third experiment, we measure the effects of variations in colour and luminance contrast on contour detection using elements that combine colour and luminance contrast. We find that varying the colour contrast of elements tends to worsen the detection of a luminance contour, as do luminance contrast variations for colour contour detection. These results suggest no special role for colour in integrating contours, and are discussed with regard to their ecological significance~.
To determine the dynamics of contour integration the temporal properties of the individual contou... more To determine the dynamics of contour integration the temporal properties of the individual contour elements were varied as well as those of the contour they form. A temporal version of a contour integration paradigm (Field, D. J., Hayes, A., & Hess, R. F. (1993) Vision Research, 33, 173-193) was used to assess these two temporal dynamics as a function of the contrast of individual elements and the curvature of the contour. The results show that the dynamics of contour integration are good when the contrast of the individual elements is modulated in time (10-30 Hz), but are poor when contour linking per se is temporally modulated (1-12 Hz). The dynamics of contour linking is not dependent on the absolute contrast of the linking elements, so long as they are visible, but does vary with the curvature of the contour. For straight contours, temporal resolution is around 6-12 Hz but falls to around 1-2 Hz for curved contours.
Is the acquisition of basic-colour terms in young children constrained? For more than a hundred y... more Is the acquisition of basic-colour terms in young children constrained? For more than a hundred years there has been substantial interest in the developmental difficulties young children experience when learning colour terms (Baldwin 1893; Darwin 1877, cited in Bornstein 1985; Nagel 1906). Initially, children may name many different colours with only one colour term and, even when they have acquired several colour terms, their naming of colour sensations is often inappropriate (eg Cruse 1977; Modreski and Goss 1969). Reliable colour naming has previously been reported to appear surprisingly late, at around 4^7 years (eg Bornstein 1985; Heider 1971; Johnson 1977; Mervis et al 1975), and may only be learned through a prolonged period of explicit instruction (Rice 1980). In contrast, the process by which young children learn the names for everyday objects appears effortless and often occurs within a single episode of hearing the word spoken in context (eg Carey 1978). The marked disparity between children's naming of everyday objects and colours suggests that the systematic process of fast mapping between novel labels and their referents that characterises early object word learning (eg Markman 1989
Shape processing involves a progression from local to global analysis. A key aspect of this is th... more Shape processing involves a progression from local to global analysis. A key aspect of this is the binding of distributed local features into an overall form followed by the extraction of the shape independently of its local contrast and spatial scales, so enabling the shape to be encoded based on its proportions without reference to its exact size or retinal location. Here we use contour-and surface-defined radial frequency (RF) patterns in a shape discrimination task, previously thought to reflect a global processing stage that has reached contrast and scale invariance. We compare performance across different spatial scales for a wide range of RF patterns (contour spatial frequencies of 0.7-10.0 cpd, pattern radii of 0.5-10.5°), and sharp-and smooth-edged surface-RF patterns, all at low contrast (5Â detection threshold). We show that shape discrimination thresholds for RF patterns have a complex series of dependencies on stimulus size (radius), contour spatial frequency (thickness) and contrast, with no scale invariance. Our results are at odds with earlier work showing no effect of radius and spatial frequency on discrimination thresholds. We show that this discrepancy can be accounted for by a differential effect of contrast on shape discrimination, with shape invariance only stabilizing at higher contrasts (10-20Â threshold).
Slant is the degree to which a surface recedes or slopes away from the observer about the horizon... more Slant is the degree to which a surface recedes or slopes away from the observer about the horizontal axis. The perception of surface slant may be derived from static monocular cues, including linear perspective and foreshortening, applied to single shapes or to multi-element textures. It is still unclear the extent to which color vision can use these cues to determine slant in the absence of achromatic contrast. Although previous demonstrations have shown that some pictures and images may lose their depth when presented at isoluminance, this has not been tested systematically using stimuli within the spatio-temporal passband of color vision. Here we test whether the foreshortening cue from surface compression (change in the ratio of width to length) can induce slant perception for single shapes for both color and luminance vision. We use radial frequency patterns with narrowband spatio-temporal properties. In the first experiment, both a manual task (lever rotation) and a visual task (line rotation) are used as metrics to measure the perception of slant for achromatic, red-green isoluminant and Scone isolating stimuli. In the second experiment, we measure slant discrimination thresholds as a function of depicted slant in a 2AFC paradigm and find similar thresholds for chromatic and achromatic stimuli. We conclude that both color and luminance vision can use the foreshortening of a single surface to perceive slant, with performances similar to those obtained using other strong cues for slant, such as texture. This has implications for the role of color in monocular 3D vision, and the cortical organization used in 3D object perception.
Purpose: In this study we examine neuroretinal function in five amblyopes, who had been shown in ... more Purpose: In this study we examine neuroretinal function in five amblyopes, who had been shown in previous functional MRI (fMRI) studies to have compromised function of the lateral geniculate nucleus (LGN), to determine if the fMRI deficit in amblyopia may have its origin at the retinal level. Methods: We used slow flash multifocal ERG (mfERG) and compared averaged five ring responses of the amblyopic and fellow eyes across a 35 deg field. Central responses were also assessed over a field which was about 6.3 deg in diameter. We measured central retinal thickness using optical coherence tomography. Central fields were measured using the MP1-Microperimeter which also assesses ocular fixation during perimetry. MfERG data were compared with fMRI results from a previous study. Results: Amblyopic eyes had reduced response density amplitudes (first major negative to first positive (N1-P1) responses) for the central and paracentral retina (up to 18 deg diameter) but not for the mid-periphery (from 18 to 35 deg). Retinal thickness was within normal limits for all eyes, and not different between amblyopic and fellow eyes. Fixation was maintained within the central 4°more than 80% of the time by four of the five participants; fixation assessed using bivariate contour ellipse areas (BCEA) gave rankings similar to those of the MP-1 system. There was no significant relationship between BCEA and mfERG response for either amblyopic or fellow eye. There was no significant relationship between the central mfERG eye response difference and the selective blood oxygen level dependent (BOLD) LGN eye response difference previously seen in these participants. Conclusions: Retinal responses in amblyopes can be reduced within the central field without an obvious anatomical basis. Additionally, this retinal deficit may not be the reason why the LGN BOLD (blood oxygen level dependent) responses are reduced for amblyopic eye stimulation.
1. A method ofproducing red-green and blue-yellow sinusoidal chromatic gratings is used which per... more 1. A method ofproducing red-green and blue-yellow sinusoidal chromatic gratings is used which permits the correction of all chromatic aberrations. 2. A quantitative criterion is adopted to choose the intensity match of the two colours in the stimulus: this is the intensity ratio at which contrast sensitivity for the chromatic grating differs most from the contrast sensitivity for a monochromatic luminance grating. Results show that this intensity match varies with spatial frequency and does not necessarily correspond to a luminance match between the colours. 3. Contrast sensitivities to the chromatic gratings at the criterion intensity match are measured as a function of spatial frequency, using field sizes ranging from 2 to 23 deg. Both blue-yellow and red-green contrast sensitivity functions have similar low-pass characteristics, with no low-frequency attenuation even at low frequencies below 041 cycles/deg. These functions indicate that the limiting acuities based on red-green and blue-yellow colour discriminations are similar at 11 or 12 cycles/deg. 4. Comparisons between contrast sensitivity functions for the chromatic and monochromatic gratings are made at the same mean luminances. Results show that, at low spatial frequencies below 0 5 cycles/deg, contrast sensitivity is greater to the chromatic gratings, consisting of two monochromatic gratings added in antiphase, than to either monochromatic grating alone. Above 0-5 cycles/deg, contrast sensitivity is greater to monochromatic than to chromatic gratings.
ABSTRACT Cross-orientation masking occurs when the detection of a test grating is masked by a sup... more ABSTRACT Cross-orientation masking occurs when the detection of a test grating is masked by a superimposed grating at an orthogonal orientation, and is widely thought to indicate the presence of "cross-channel" interactions mediating contrast normalization. In achromatic vision, modeling and psychophysical experiments have suggested that there are at least two routes to cross-orientation suppression prior to binocular summation: a within-eye (monocular) pathway that is non-adaptable, and an interocular (dichoptic) adaptable pathway that is cortical and mediates mutual suppression between the eyes (Baker et al., Neuroscience, 146, 2007: Meese & Hess, 2004). Here we develop this two-stage model and test its application to color vision. Test and mask stimuli were red-green isoluminant Gabors presented orthogonally. TvC masking functions were obtained for three spatial frequencies (0.375, 0.75 & 1.5cpd at 2Hz) for monocular, binocular and dichoptic presentations in four subjects. We generalized the two-stage model so that it could be used to fit the monocular, dichoptic and binocular TvC functions. We determined the weight of suppression parameters for the monocular and dichoptic suppression, with the remaining parameters fixed on the basis of known values. We find that the two-stage model is a good fit to the chromatic data, supporting the idea that color and achromatic vision use the same two routes to cross-orientation suppression. The dichoptic pathway shows greater suppression than monocular in color vision. It is also unselective as it can be activated by achromatic or chromatic contrast (Mullen et al., 2012). We find no effect of spatial frequency on either pathway.
The apparent contrast of a plaid is a reflection of the neural relationship between the responses... more The apparent contrast of a plaid is a reflection of the neural relationship between the responses to its two orthogonal component gratings. To investigate the perceived contrast summation of the responses to component gratings in plaids, we compared the apparent contrasts of monocular plaids to a component grating presented alone across chromaticity and spatial frequency. Observers performed a contrast-matching task for red-green color and luminance stimuli at low-and medium-spatial frequencies. Using the measured points of subjective equality between plaids and gratings, we evaluate perceived contrast summation across conditions, which may vary between 1 (no summation) and 2 (full summation). We show that achromatic plaids have higher perceived contrast summation than chromatic plaids. The greatest difference occurs at the medium-spatial frequency, with summation highest for achromatic plaids (1.87) and lowest for chromatic plaids (1.49), while at low-spatial frequencies, there is a smaller summation difference between achromatic (1.72) and chromatic (1.65) plaids. These results are consistent with recent theories of distinct cross-orientation suppression and summation mechanisms in color and luminance vision. Two control experiments for binocular versus monocular viewing, and the overall size of the stimulus patches did not reveal any differences from our main results.
We measure the orientation tuning of red-green colour and luminance vision at low (0.375 c/deg) a... more We measure the orientation tuning of red-green colour and luminance vision at low (0.375 c/deg) and mid (1.5 c/deg) spatial frequencies using the low-contrast psychophysical method of subthreshold summation. Orientation bandwidths of the underlying neural detectors are found using a model involving Minkowski summation of the rectified outputs of a bank of oriented filters. At 1.5 c/deg, we find orientation-tuned detectors with similar bandwidths for chromatic and achromatic contrast. At 0.375 c/deg, orientation tuning is preserved with no change in bandwidth for achromatic stimuli, however, for chromatic stimuli orientation tuning becomes extremely broad, compatible with detection by non-oriented colour detectors. A non-oriented colour detector, previously reported in single cells in primate V1 but not psychophysically in humans, can transmit crucial information about the color of larger areas or surfaces whereas orientation-tuned detectors are required to detect the colour or luminance edges that delineate an object's shape.
In our visual world we can distinguish with ease between chromatic and luminance contrasts. Howev... more In our visual world we can distinguish with ease between chromatic and luminance contrasts. However, in our retinae most neurones are responsive to both chromatic and luminance changes and therefore send ambiguous or 'multiplexed' information to the higher visual centres. Psychophysical evidence suggests that some cortical process must subsequently separate out this information into its chromatic and luminance components. The purpose of this communication is to review and critically evaluate the different existing schemes for doing this. To assist in this evaluation a linear systems analysis is employed in which model cortical neurones are imputed with the property of providing information about either colour or luminance. It is concluded that there is currently no unified scheme available to explain a separation of colour and luminance information in the visual system. Some theoretical considerations and most promising approaches to solving the problem are noted, but it is suggested that there may be definite limits to the ability of the visual system to achieve complete separation of colour and luminance from the retinal signal.
We have investigated the spatial transfer characteristics of the mechanisms sensitive to color in... more We have investigated the spatial transfer characteristics of the mechanisms sensitive to color in the human visual system using a method of shnultaneous spatial masking with isol~uaut chromatic sthuuli. The test sthnuli were Gaussian enveloped &-green gratings of three spatial frequencies in the lowpasu region of the color domain (0.25, 0.5 and 1 c/deg). The maskhrg stimuli were redgreen gratings at the orientation and phase of the test, presented at the same spatial frequency, and at & 1, and + 2 octaves from its spatial frequency. We obtained test contrast &e&old as a function of mask contrast for a wide range of mask contrasts (TvC functions). Tuuhrg fuuctiorw were derived from linear fits of the masking data, by taking the mask contrast that doubled the mhdmum test thre&old at each spatial frequency. Chromatic tuning functions show handpass characteristics for all teat spatial frequencies examined with an average full bandwidth at half-height of 2.6 octaves, which is shnilar to the huninance bandwidths obtained under comparable conditions. Thus, our results suggest that the color contrast sensitivity function is the upper envelope of a range of bandpass mechanisms whose peaks extend to very low spatial frequencies.
In this study, we consider how colour contrast can be used to integrate form and how it interacts... more In this study, we consider how colour contrast can be used to integrate form and how it interacts with luminance contrast in the task. The performance of form integration was assessed by measuring the detection of a winding "contour" of aligned gabor elements embedded in a background of randomly oriented gabors, using both luminance and isoluminant (red/green) contrast. Performance on the task improves with gabor element contrast, and identical performance for colour and luminance contour detection is achieved at high screen contrasts, showing that coiour is able to support a complex form integration task. In a second experiment, we investigate whether colour and luminance contrast can be combined in contour integration by measuring the detection of a path with alternating isoluminant colour and luminance elements. We find that contour detection uses both colour and luminance information cooperatively, but performance is much poorer than would be expected from a single common contour integration process which fails to distinguish the two types of contrast. This suggests that there are specific contour integration processes for colour and luminance. In a third experiment, we measure the effects of variations in colour and luminance contrast on contour detection using elements that combine colour and luminance contrast. We find that varying the colour contrast of elements tends to worsen the detection of a luminance contour, as do luminance contrast variations for colour contour detection. These results suggest no special role for colour in integrating contours, and are discussed with regard to their ecological significance~.
To determine the dynamics of contour integration the temporal properties of the individual contou... more To determine the dynamics of contour integration the temporal properties of the individual contour elements were varied as well as those of the contour they form. A temporal version of a contour integration paradigm (Field, D. J., Hayes, A., & Hess, R. F. (1993) Vision Research, 33, 173-193) was used to assess these two temporal dynamics as a function of the contrast of individual elements and the curvature of the contour. The results show that the dynamics of contour integration are good when the contrast of the individual elements is modulated in time (10-30 Hz), but are poor when contour linking per se is temporally modulated (1-12 Hz). The dynamics of contour linking is not dependent on the absolute contrast of the linking elements, so long as they are visible, but does vary with the curvature of the contour. For straight contours, temporal resolution is around 6-12 Hz but falls to around 1-2 Hz for curved contours.
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Papers by Kathy Mullen