Chromatic and achromatic vision of macaques: role of the P pathway

Chromatic and achromatic contrast sensitivity were measured in a human observer, 2 normal macaque monkeys, and 3 monkeys with severe toxicant-induced damage to the parvocellular projecting retinogeniculate pathway (P cell-deficient monkeys). Damage to the P pathway was produced by the oral administration of acrylamide monomer (Eskin and Merigan, 1986). Contrast sensitivity was measured in all subjects with isochromatic luminance gratings, as well as isoluminant chromatic gratings, modulated along several directions of a color space that represents color-opponent and luminance contrast (Krauskopf et al., 1986). The chromatic and achromatic sensitivity of the control monkeys was virtually identical to that of the human observer. Chromatic sensitivity of the P cell-deficient monkeys, measured at a low spatial frequency (0.3 c/deg), along a constant-blue color axis, was 0.9-1.5 log units lower than that of controls. Similar losses were seen along a tritanopic confusion axis and along 2 intermediate axes of color direction. Chromatic thresholds measured at higher spatial frequency (2.0 c/deg) were similarly reduced. Counterphase-modulated chromatic gratings were used to test color sensitivity over a range of temporal frequencies up to 15 Hz, and the loss of color vision was substantial over the entire range of frequencies. The luminance contrast sensitivity of the P cell-deficient monkeys for stationary gratings decreased after exposure by 0.5-0.8 log units. These results indicate that the chromatic and achromatic spatial vision of macaques is very similar to that of humans. They also suggest that the P pathway plays an important role in macaque chromatic sensitivity at all spatial frequencies, as well as achromatic sensitivity at high spatial and lower temporal frequencies.     

Neural basis of photo/chromatic sensitivity in adolescence

Purpose: To determine a psychophysiological basis for age visual sensitivity to chromatic and achromatic stimuli. Methods: We investigated the effects of achromatic and four isoluminant color combinations (blue/red, blue/green, green/red, and blue/yellow), luminance ratio changes in color combinations (blue/red; 1:1, 3:4, 4:3) and contrast changes (3 to 100%) on steady‐state electroretinograms (ERGs) and visual evoked potentials (VEPs) in 32 healthy teenagers and 30 young adults. Results: We found that (1) dual peaks at 9 and 18 Hz with a dip at 12 Hz were observed in VEPs with all isoluminant color combinations, (2) VEP responses were significantly enhanced and the 12‐Hz dip became unclear with luminance ratio changes between two colors with a nonantagonistic relationship (blue/red), and (3) VEP amplitudes were significantly increased when the contrast was increased. These characteristics were more evident in teenagers than young adults; however, ERGs were qualitatively similar between the two groups. Discussion: The visual cortex is differently modulated by different color‐luminance combinations, and higher sensitivity to color‐luminance combinations in the visual cortex in teenagers is responsible for the high prevalence of photo/chromatic sensitivity in adolescence.     

Changes in pattern electroretinograms to equiluminant red-green and blue-yellow gratings in patients with early Parkinson's disease.

In Parkinson's disease (PD), the luminance pattern electroretinogram (PERG) is reported to be abnormal, indicating dysfunction of retinal ganglion cells (RGCs). To determine the vulnerability of different subpopulations of RGCs in PD patients, the authors recorded the PERG to stimuli of chromatic (red-green [R-G] and blue-yellow [B-Y]) and achromatic (yellow-black [Y-Bk]) contrast, known to emphasize the contribution of parvocellular, koniocellular, and magnocellular RGCs, respectively. Subjects were early PD patients (n = 12; mean age, 60.1 +/- 8.3 years; range, 46 to 74 years) not undergoing treatment with levodopa and age-sex-matched controls (n = 12). Pattern electroretinograms were recorded monocularly in response to equiluminant R-G, B-Y, and Y-Bk horizontal gratings of 0.3 c/deg and 90% contrast, reversed at 1Hz, and presented at a viewing distance of 24 cm (59.2 x 59 degree field). In PD patients, the PERG amplitude was significantly reduced (by 40 to 50% on average) for both chromatic and luminance stimuli. Pattern electroretinogram latency was significantly delayed (by about 15 ms) for B-Y stimuli only. Data indicate that, in addition to achromatic PERGs, chromatic PERGs are altered in PD before levodopa therapy. Overall, chromatic PERGs to B-Y equiluminant stimuli exhibited the largest changes. Data are consistent with previous findings in PD, showing that visual evoked potentials (VEP) to B-Y chromatic stimuli are more delayed than VEPs to R-G and achromatic stimuli. The results suggest that the koniocellular subpopulation of RGCs may be particularly vulnerable in early stages of Parkinson's disease.     

The effect of spatial frequency on chromatic and achromatic steady-state visual evoked potentials.

OBJECTIVE: Little is known about the physiological properties of the major components of steady-state visual evoked potentials (VEPs). Based on the hypothesis that isoluminant color and high contrast pattern differentially activate the parvo- and magnocellular pathways, we studied difference in spatial frequency function between chromatic and achromatic VEPs to sinusoidal gratings. METHODS: Steady-state VEPs to isoluminant chromatic (red-green) and high contrast (90%) achromatic (black-white) sinusoidal gratings with nine spatial frequencies (0.5 to 8.0 cycles/degrees (cpd)) at 4 Hz (8 reversals/s) were recorded in 13 normal subjects. VEPs were Fourier analyzed to obtain phase and amplitude of the second (2F) and fourth (4F) harmonic responses. RESULTS: The 2F amplitude of chromatic VEPs decreased above 4.0 cpd in a low-pass function while that of achromatic VEPs showed a band-pass function with a peak at 4.0 cpd. The 4F amplitude of chromatic VEPs was not affected significantly by spatial frequency whereas that of achromatic VEPs exhibited a high-pass function. The phases of 2F and 4F showed a non-monotonic function of spatial frequency in both chromatic and achromatic stimuli with peaks at middle spatial frequencies. CONCLUSION: Chromatic and achromatic visual stimuli differently affected 2F and 4F components, which thus suggests that 2F and 4F components are generated from different neuronal subgroups largely in the parvocellular pathway.     

The contribution of color to motion processing in Macaque middle temporal area.

The chromatic properties of an image yield strong cues for object boundaries and thus hold the potential to facilitate the detection of object motion. The extent to which cortical motion detectors exploit chromatic information, however, remains a matter of debate. To address this further, we quantified the strength of chromatic input to directionally selective neurons in the middle temporal area (MT) of macaque cerebral cortex using an equivalent luminance contrast (EqLC) paradigm. This paradigm, in which two sinusoidal gratings, one heterochromatic and the other achromatic, are superimposed and moved in opposite directions, allows the sensitivity of motion detectors to heterochromatic stimuli to be quantified and expressed relative to the benchmark of sensitivity for a luminance-defined stimulus. The results of these experiments demonstrate that the chromatic contrast in a moving red-green heterochromatic grating strongly influences directional responses in MT when the luminance contrast in that same grating is relatively low; for such stimuli, EqLC is at least 5%. When luminance contrast is added to the heterochromatic grating, however, EqLC wanes sharply and becomes negative (-4%) when luminance contrast is sufficiently high (>17-23%). Thus, the chromatic properties of an object appear to confer little or no benefit to motion processing by MT neurons when sufficient luminance contrast concurrently exists. These data support a simple model in which chromatic motion processing in MT is almost exclusively determined by magnocellular input. Additionally, a comparison of neuronal and psychophysical data suggests that MT may not be the sole contributor to the perceptual experience elicited by motion of heterochromatic patterns, or that only a subset of MT neurons serve this function.     

Luminance and chromatic contrast sensitivity in dyslexia: the magnocellular deficit hypothesis revisited

The hypothesis of a magnocellular channel deficit in dyslexia was tested. Subjects were 10-year-old dyslexics and normal readers. Psychophysical thresholds for luminance and chromatic contrasts were estimated using black and white and red and green sinusoidal gratings of various spatial frequencies, presented in static and dynamic conditions (drift and reversal). Significant group differences were found for luminance contrast, with a higher sensitivity in dyslexics. No group differences were obtained for chromatic contrast. High luminance contrast sensitivity correlated with low reading and writing skills. The typical finding of an increase contrast sensitivity to low spatial frequency gratings, due to their dynamic presentations, was absent in dyslexics. The results provide support for the magnocellular deficit hypothesis. The pattern of this deficit, however, is much more complex than that emerging from previous research.     

Visual-evoked potentials to onset of chromatic red-green and blue-yellow gratings in Parkinson's disease never treated with L-dopa.

The differential dysfunction of chromatic and achromatic visual pathways in early Parkinson's disease (PD) was evaluated by means of visual-evoked potentials (VEPs) recorded in 12 patients (mean age 60.1 +/- 8.3 years; range 46 to 74 years) in the early stages of PD and not yet undergoing treatment with L-dopa, and in 12 age-matched controls. Visual stimuli were full-field (14 deg) equiluminant red-green (R-G), blue-yellow (B-Y), and black-white (B-W) sinusoidal gratings of two cycles per degree, presented in onset (300 milliseconds)--offset (700 milliseconds) mode, at two contrast (K) levels (90% and 25%). The VEP mean latencies were significantly more delayed in PD patients than in controls for chromatic than for luminance stimuli, in particular for B-Y stimuli of low contrast (K90%: B-W = 6.6 milliseconds, R-G = 3.34 milliseconds, B-Y = 15.48 milliseconds; K25%: B-W = 7.8 milliseconds, R-G = 14.8 milliseconds, B-Y = 28.9). Latencies of chromatic VEPs were more variable that achromatic VEP latencies in both normal subjects and PD patients. Therefore, the frequency of latency abnormalities (within 30%) was not significantly different for the three visual stimuli. Our results show that, in addition to achromatic VEPs, chromatic VEPs are impaired in early PD patients not yet undergoing L-dopa therapy, indicating an acquired color deficiency in these patients. The greater delay for the B-Y VEPs suggests a higher vulnerability of visual blue-cone pathway in the early stages of the disease. However, the overall sensitivity of chromatic VEPs in detecting early visual impairment in PD is comparable with that of achromatic VEPs.     

Vision abnormalities in young children exposed prenatally to organic solvents

Despite an accumulating body of evidence demonstrating that the visual system is an important target for organic solvent toxicity in adults, little attention has been paid to the visual functioning of children with prenatal exposure to organic solvents. The present study aimed to: (1) determine prospectively whether prenatal solvent exposure increases the risk of visual deficits in infants and (2) assess the relationship between estimates of exposure level and integrity of visual responses. A sample of 21 infants born to women who were occupationally exposed to solvents during pregnancy was compared with 27 non-exposed age-matched control infants. All mothers were recruited from Motherisk, an antenatal counseling service in Toronto, Canada. Contrast sensitivity and grating acuity were assessed using a sweep visual evoked potential (VEP) technique whereas chromatic- and achromatic mechanisms were assessed using a transient VEP technique. Exposure level was estimated from questionnaire data obtained during pregnancy. Testers were masked to exposure status. Results showed a significant reduction in contrast sensitivity in the low and intermediate spatial frequency range in solvent-exposed infants compared to controls (p<0.001). With respect to grating acuity, there was a significant effect of exposure level, with children in the high exposed having reduced grating acuity compared with children in the low exposed group (p<0.025) and controls (p=0.02). Regarding color vision, 26.3% of infants in the exposed group versus 0% of the controls produced abnormal VEP responses to the red-green onset stimulus (p<0.01), but not to either blue-yellow or achromatic stimuli. No differences were found with respect to latency or amplitude of chromatic and achromatic response. These findings suggest that prenatal solvent exposure is associated with selective visual deficits, including reduced contrast sensitivity and abnormal red-green vision. Increasing levels of exposure may lead to further visual deficits affecting grating acuity. These findings support the need for a re-evaluation of current occupational exposure standards for pregnant women.     

Chromatic mechanisms in striate cortex of macaque

We measured the responses of 305 neurons in striate cortex to moving sinusoidal gratings modulated in chromaticity and luminance about a fixed white point. Stimuli were represented in a 3-dimensional color space defined by 2 chromatic axes and a third along which luminance varied. With rare exceptions the chromatic properties of cortical neurons were well described by a linear model in which the response of a cell is proportional to the sum (for complex cells, the rectified sum) of the signals from the 3 classes of cones. For each cell there is a vector passing through the white point along which modulation gives rise to a maximal response. The elevation (theta m) and azimuth (phi m) of this vector fully describe the chromatic properties of the cell. The linear model also describes neurons in l.g.n. (Derrington et al., 1984), so most neurons in striate cortex have the same chromatic selectivity as do neurons in l.g.n. However, the distributions of preferred vectors differed in cortex and l.g.n.: Most cortical neurons preferred modulation along vectors lying close to the achromatic axis and those showing overt chromatic opponency did not fall into the clearly defined chromatic groups seen in l.g.n. The neurons most responsive to chromatic modulation (found mainly in layers IVA, IVC beta, and VI) had poor orientation selectivity, and responded to chromatic modulation of a spatially uniform field at least as well as they did to any grating. We encountered neurons with band-pass spatial selectivity for chromatically modulated stimuli in layers II/III and VI. Most had complex receptive fields. Neurons in layer II/III did not fall into distinct groups according to their chromatic sensitivities, and the chromatic properties of neurons known to lie within regions rich in cytochrome oxidase appeared no different from those of neurons in the interstices. Six neurons, all of which resembled simple cells, showed unusually sharp chromatic selectivity.     

Behavioural and electrophysiological chromatic and achromatic contrast sensitivity in an achromatopsic patient.

OBJECTIVES--In cases of incomplete achromatopsia it is unclear whether residual visual function is mediated by intact striate cortex or results from incomplete lesions to extrastriate cortical visual areas. A patient with complete cerebral achromatopsia was tested to establish the nature of his residual vision and to determine the integrity of striate cortex function. METHODS--Behavioural contrast sensitivity, using the method of adjustment, and averaged visually evoked cortical potentials were measured to sinusoidally modulated chromatic and achromatic gratings in an achromatopsic patient and a normal observer. Eye movements were measured in the patient using a Skalar infrared monitoring system. RESULTS--The patient's chromatic contrast sensitivity was normal, indicating that despite his dense colour blindness his occipital cortex still processed information about spatial variations in hue. His sensitivity to achromatic gratings was depressed particularly at high spatial frequencies, possibly because of his jerk nystagmus. These behavioural results were reinforced by the nature of visually evoked responses to chromatic and achromatic gratings, in which total colour blindness coexisted with an almost normal cortical potential to isoluminant chromatic gratings. CONCLUSIONS--The results show that information about chromatic contrast is present in some cortical areas, and coded in a colour-opponent fashion, in the absence of any perceptual experience of colour.     

Independent patterns of damage to retinocortical pathways in multiple sclerosis without a previous episode of optic neuritis

Asymptomatic visual loss is a feature of multiple sclerosis (MS) but its relative impact on distinct retinocortical pathways is still unclear. The goal of this work was to investigate patterns of subclinical visual impairment in patients with MS with and without clinically associated previous optic neuritis (ON). We have used functional methods that assess parvo-, konio- and magnocellular pathways in order to compare pathophysiological mechanisms of damage in a population of 44 subjects with MS (87 eyes), with and without a previous episode of ON. These methods included chromatic contrast sensitivity across multiple chromatic axes (Cambridge Colour Test–parvo/konio pathways), perimetric achromatic contrast sensitivity for the magno pathway [frequency doubling technique (FDT)] and pattern visual evoked potentials (VEP). These measures were correlated with field sensitivity measures obtained using conventional automated static perimetry (ASP) and were also compared with conventional clinical chromatic/achromatic contrast sensitivity chart-based measures. We have found evidence for uncorrelated damage of all retinocortical pathways only in patients with MS without ON. VEP evidence for axonal damage was found in this group supporting the emerging notion of axonal damage even in sub-clinical stages of ON/MS pathophysiology. Only in this group was significant correlation of functional measures with disease stage observed, suggesting that distinct pathophysiological milestones are present before and after ON has occurred.     

Temporal properties of colour opponent receptive fields in the cat lateral geniculate nucleus

The primordial form of mammalian colour vision relies on opponent interactions between inputs from just two cone types, ‘blue’ (S‐) and ‘green’ (ML‐) cones. We recently described the spatial receptive field structure of colour opponent blue‐ON cells from the lateral geniculate nucleus of cats. Functional inputs from the opponent cone types were spatially coextensive and equally weighted, supporting their high chromatic and low achromatic sensitivity. Here, we studied relative cone weights, temporal frequency tuning and visual latency of cat blue‐ON cells and non‐opponent achromatic cells to temporally modulated cone‐isolating and achromatic stimuli. We confirmed that blue‐ON cells receive equally weighted antagonistic inputs from S‐ and ML‐cones whereas achromatic cells receive exclusive ML‐cone input. The temporal frequency tuning curves of S‐ and ML‐cone inputs to blue‐ON cells were tightly correlated between 1 and 48 Hz. Optimal temporal frequencies of blue‐ON cells were around 3 Hz, whereas the frequency optimum of achromatic cells was close to 10 Hz. Most blue‐ON cells showed negligible response to achromatic flicker across all frequencies tested. Latency to visual stimulation was significantly greater in blue‐ON than in achromatic cells. The S‐ and ML‐cone responses of blue‐ON cells had on average, similar latencies to each other. Altogether, cat blue‐ON cells showed remarkable balance of opponent cone inputs. Our results also confirm similarities to primate blue‐ON cells suggesting that colour vision in mammals evolved on the basis of a sluggish pathway that is optimized for chromatic sensitivity at a wide range of spatial and temporal frequencies.     

Deficits in achromatic phantom contour perception in poor readers

In a previous study [Sperling, A. J., Lu, Z. L., Manis, F. R., & Seidenberg, M. S. (2003). Selective deficits in magnocellular processing: A "phantom contour" study. Neuropsychologia, 41, 1422-1429] we found that dyslexic children were relatively slower in processing achromatic phantom contours. The maximum temporal frequency at which they could identify achromatic phantom contours was correlated with reading ability and orthographic skill in particular. Here we investigated whether similar deficits could be identified in adults. Poor readers were chosen who scored below the 25th percentile on either a standardized test of word identification or nonword pronunciation. Good readers were chosen who scored above the 40th percentile on both reading tasks. We replicated the findings of the child study: poor readers had slower processing in the achromatic version of the task, but not in the chromatic version. Achromatic performance correlated with several measures of reading and reading-related skills, including exception word reading and phonological awareness. We discuss the possibility that the deficits may indicate impairment in noise exclusion that is more readily apparent at higher temporal frequencies.     

Parvocellular pathway impairment in autism spectrum disorder: Evidence from visual evoked potentials

In humans, visual information is processed via parallel channels: the parvocellular (P) pathway analyzes color and form information, whereas the magnocellular (M) stream plays an important role in motion analysis. Individuals with autism spectrum disorder (ASD) often show superior performance in processing fine detail, but impaired performance in processing global structure and motion information. To date, no visual evoked potential (VEP) studies have examined the neural basis of atypical visual performance in ASD. VEPs were recorded using 128-channel high density EEG to investigate whether the P and M pathways are functionally altered in ASD. The functioning of the P and M pathways within primary visual cortex (V1) were evaluated using chromatic (equiluminant red–green sinusoidal gratings) and achromatic (low contrast black–white sinusoidal gratings) stimuli, respectively. Unexpectedly, the N1 component of VEPs to chromatic gratings was significantly prolonged in ASD patients compared to controls. However, VEP responses to achromatic gratings did not differ significantly between the two groups. Because chromatic stimuli preferentially stimulate the P-color but not the P-form pathway, our findings suggest that ASD is associated with impaired P-color pathway activity. Our study provides the first electrophysiological evidence for P-color pathway impairments with preserved M function at the V1 level in ASD.     

Luminance,but not chromatic visual pathways,mediate amplification of conditioned danger signals in human visual cortex

Complex organisms rely on experience to optimize the function of perceptual and motor systems in situations relevant to survival. It is well established that visual cues reliably paired with danger are processed more efficiently than neutral cues, and that such facilitated sensory processing extends to low levels of the visual system. The neurophysiological mechanisms mediating biased sensory processing, however, are not well understood. Here we used grating stimuli specifically designed to engage luminance or chromatic pathways of the human visual system in a differential classical conditioning paradigm. Behavioral ratings and visual electroencephalographic steady‐state potentials were recorded in healthy human participants. Our findings indicate that the visuocortical response to high‐spatial‐frequency isoluminant (red–green) grating stimuli was not modulated by fear conditioning, but low‐contrast, low‐spatial‐frequency reversal of grayscale gratings resulted in pronounced conditioning effects. We conclude that sensory input conducted via the chromatic pathways into retinotopic visual cortex has limited access to the bi‐directional connectivity with brain networks mediating the acquisition and expression of fear, such as the amygdaloid complex. Conversely, luminance information is necessary to establish amplification of learned danger signals in hierarchically early regions of the visual system.     

Chromatic pattern-reversal electroretinograms (ChPERGs) are spared in multiple system atrophy compared with Parkinson’s disease

Abstract Idiopathic Parkinson’s disease (IPD) patients have abnormal visual evoked potentials (VEPs) and pattern electroretinograms (PERGs), attributed to dopaminergic transmission deficiency in visual pathway, probably the retina. VEP abnormalities are not reported in multiple system atrophy (MSA). The aim of this study was to investigate and compare chromatic (Ch) red-green (R-G) and blue-yellow (B-Y), and luminance yellow-black (Y-Bk) PERGs in patients with MSA and IPD. We investigated 6 MSA patients (mean age: 62±7.4 years) not undergoing any pharmacological treatment, as well as 12 early IPD patients (mean age: 60.1±8.3 years) and 12 age-matched normal observers. ChPERGs were recorded monocularly in response to full-field equiluminant R-G, B-Y and Y-Bk horizontal gratings. In MSA only responses to R-G stimuli showed minimal insignificant changes (slight but not significant amplitude reduction without any significant latency delay); no significant abnormality was detected for B-Y and luminance Y-Bk stimuli. By contrast, in IPD all responses were reduced in amplitude and delayed in latency, above all for B-Y stimuli. Present data indicate that both chromatic and achromatic PERGs are virtually unaffected in MSA, whereas in early IPD they are clearly impaired, suggesting different pathogenic retinal mechanisms and a useful simple tool for distinguishing MSA from IPD.     

Chromatic modulation of luminance visual evoked potential latencies in healthy subjects and patients with mild vision disorders.

OBJECTIVE: To study whether and how color modulates luminance visual evoked potentials (VEPs). METHODS: We studied pattern-reversal luminance VEPs to red/black and blue/black checkerboards with identical luminance contrast values (mixed luminance and chromatic components) (isocontrast color VEP, in brief, IVEPs) in 25 healthy subjects and two groups of patients with mild vision disorders (23 with glaucoma and 25 with optic neuritis). We then compared these with the standard color VEPs to pure chromatic contrast red/green and blue/yellow gratings (CVEPs). RESULTS: In healthy subjects, VEPs to red/black checkerboards and red/green gratings were slower than those obtained with blue/black checkerboards and blue/yellow gratings. Both procedures (IVEPs and CVEPs) differentiated patients with vision disorders from healthy subjects and distinguished between the two different vision disorders. Red/black checkerboards and red-green gratings elicited slower VEPs in patients with optic neuritis and blue/black checkerboards and blue/yellow gratings elicited slower VEPs in patients with glaucoma. IVEPs appeared more stable and ample than CVEPs. The contrast indices normalized CVEP and IVEP latencies in the same subject and showed a positive correlation between CVEP and IVEP latencies in healthy subjects and in patients with optic neuritis, but not in patients with glaucoma. CONCLUSIONS: Our study confirms the usefulness of CVEPs in detecting and differentiating mild vision disorders. IVEPs to colored pattern-reversal luminance checkerboards are equally effective in distinguishing between various vision disorders possibly because colors can modulate VEP latencies to luminance contrast stimuli. SIGNIFICANCE: IVEPs can be useful in differentiating the various vision disorders and are easier than CVEPs to test in a routine clinical setting.     

The effects of parvocellular lateral geniculate lesions on the acuity and contrast sensitivity of macaque monkeys

The effects of ablating the visual pathway that passes through the parvocellular (dorsal) LGN were tested in 2 macaque monkeys by measuring acuity and both luminance and chromatic contrast sensitivity. Thresholds were tested monocularly before and after ibotenic acid was used to lesion parvocellular layers 4 and 6 of the contralateral geniculate. The injections were centered at the representation of 6 degrees in the temporal field on the horizontal meridian, and vision was tested with localized stimuli at this location. In addition, in one of the monkeys, a lesion was made in magnocellular layer 1 of the opposite geniculate, and the same thresholds were tested. Physiological and anatomical reconstructions demonstrated complete destruction of the target layers in 1 parvocellular lesions and in the magnocellular lesion, and sparing of the nontarget layers in the tested region. Parvocellular lesions caused a 3-4-fold reduction in visual acuity within the affected part of the visual field, while the magnocellular lesion did not affect acuity. Both luminance and chromatic contrast sensitivity, tested with stationary gratings of 2 c/degree, were severely reduced by parvocellular lesions, but not affected by the magnocellular lesion. However, when luminance contrast sensitivity was tested with 1 c/degree gratings, counterphase modulated at 10 Hz, it was reduced by both parvocellular and magnocellular lesions. This study demonstrates that the parvocellular pathway dominates chromatic vision, acuity, and contrast detection at low temporal and high spatial frequencies, while the magnocellular pathway may mediate contrast detection at higher temporal and lower spatial frequencies.     

Attentional modulation of fMRI responses in human V1 is consistent with distinct spatial maps for chromatically defined orientation and contrast

Attending to different stimulus features such as contrast or orientation can change the pattern of neural responses in human V1 measured with fMRI. We show that these pattern changes are much more distinct for colored stimuli than for achromatic stimuli. This is evidence for a classic model of V1 functional architecture in which chromatic contrast and orientation are coded in spatially distinct neural domains, while achromatic contrast and orientation are not.     

Development of visual function: a neurophysiological point of view

Recent findings on the development of visual function in children are summarized. First, visual evoked potentials (VEPs) in normal school children, following two types of visual stimuli (pattern reversal and light emitting diode stimulation) by transient and steady-state stimulation, are presented. Reproducible VEPs with the 4 stimulation conditions can be obtained. Transient and steady-state methods provide complementary results. Second, mechanisms for photosensitive epilepsy (PSE) are discussed. We recorded flicker VEPs to different color combinations and quantified the effects of changes in color and luminance combinations. Two amplitude peaks (9 and 18 Hz) were observed for all kinds of isoluminant color combination stimuli against temporal frequency. In addition, this characteristic was modulated by luminance. Normal children showed much higher sensitivity to contrast changes and color combination compared with normal young adults, which may be responsible for PSE in childhood. Third, cognitive function for facial expression of normal children and adults is mentioned. For Chernoff's faces showing anger and sadness produced by computer, children showed higher scores compared with adults, suggesting higher sensitivity for facial expression. Knowledge of developmental changes in children allows us to understand the maturational and degenerative changes of the central nervous system.