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.     

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.     

Equiluminant red-green and blue-yellow VEPs in multiple sclerosis.

The primate visual system is composed by two color-opponent pathways--red-green (R-G) and blue-yellow (B-Y)--subserved by the so-called parvo- and koniocellular streams respectively. The authors' aim was to compare the relative involvement of chromatic visual subsystems in multiple sclerosis (MS). In 30 MS patients with different forms of MS they recorded visual evoked potentials (VEPs) to onset (300 msec) and offset (700 msec) of equiluminant R-G and B-Y sinusoidal gratings of different contrast (90% and 25%). Equiluminance was established psychophysically by establishing the R-G and the B-Y color ratio at which chromatic gratings alternating at 15 and 10 Hz respectively had minimum visibility. The negative wave at stimulus onset with a peak latency of 120 to 160 msec was evaluated. Ordinary VEPs to luminance (LUM) contrast (black-white reversing checkerboards of 15' check size and 50% contrast) were also recorded for comparison. Latencies of R-G VEPs were abnormal in 53.3% and 58.3% of patients at 90% and 25% contrast respectively, whereas abnormal B-Y VEPs were 56.6% and 48.3%. Latencies of LUM VEPs were abnormal in 45% of patients. Interocular latency asymmetries were abnormal in 59.2% and 33.3% of patients for R-G, and 51.8% and 62.9% for B-Y. Latency asymmetries for LUM VEP were abnormal in 46.4% of patients. The higher rate of VEP abnormalities found with equiluminant chromatic stimuli compared with achromatic stimuli confirms the general vulnerability of color-opponent visual pathways in MS, even if the number of patients with abnormal findings was not significantly different when both test conditions were compared. VEPs to R-G and B-Y equiluminant stimuli appear to be involved approximately to the same extent.     

Contrast sensitivity for motion detection and direction discrimination in adolescents with autism spectrum disorders and their siblings

The magnocellular (M) pathway hypothesis proposes that impaired visual motion perception observed in individuals with Autism Spectrum Disorders (ASD) might be mediated by atypical functioning of the subcortical M pathway, as this pathway provides the bulk of visual input to cortical motion detectors. To test this hypothesis, we measured luminance and chromatic contrast sensitivity, thought to tap M and Parvocellular (P) pathway processing, respectively. We also tested the hypothesis that motion processing is impaired in ASD using a novel paradigm that measures motion processing while controlling for detectabilty. Specifically, this paradigm compares contrast sensitivity for detection of a moving grating with contrast sensitivity for direction-of-motion discrimination of that same moving grating. Contrast sensitivities from adolescents with ASD were compared to typically-developing adolescents, and also unaffected siblings of individuals with ASD (SIBS). The results revealed significant group differences on P, but not M, pathway processing, with SIBS showing higher chromatic contrast sensitivity than both participants with ASD and TD participants. This atypicality, unique to SIBS, suggests the possible existence of a protective factor in these individuals against developing ASD. The results also revealed impairments in motion perception in both participants with ASD and SIBS, which may be an endophenotype of ASD. This impairment may be driven by impairments in motion detectors and/or by reduced input from neural areas that project to motion detectors, the latter possibility being consistent with the notion of reduced connectivity between neural areas in ASD.     

Selectivity of human retinotopic visual cortex to S-cone-opponent, L/M-cone-opponent and achromatic stimulation

Our aim was to make a quantitative comparison of the response of the different visual cortical areas to selective stimulation of the two different cone-opponent pathways [long- and medium-wavelength (L/M)- and short-wavelength (S)-cone-opponent] and the achromatic pathway under equivalent conditions. The appropriate stimulus-contrast metric for the comparison of colour and achromatic sensitivity is unknown, however, and so a secondary aim was to investigate whether equivalent fMRI responses of each cortical area are predicted by stimulus contrast matched in multiples of detection threshold that approximately equates for visibility, or direct (cone) contrast matches in which psychophysical sensitivity is uncorrected. We found that the fMRI response across the two colour and achromatic pathways is not well predicted by threshold-scaled stimuli (perceptual visibility) but is better predicted by cone contrast, particularly for area V1. Our results show that the early visual areas (V1, V2, V3, VP and hV4) all have robust responses to colour. No area showed an overall colour preference, however, until anterior to V4 where we found a ventral occipital region that has a significant preference for chromatic stimuli, indicating a functional distinction from earlier areas. We found that all of these areas have a surprisingly strong response to S-cone stimuli, at least as great as the L/M response, suggesting a relative enhancement of the S-cone cortical signal. We also identified two areas (V3A and hMT+) with a significant preference for achromatic over chromatic stimuli, indicating a functional grouping into a dorsal pathway with a strong magnocellular input.     

Differential inhibition of chromatic and achromatic perception by transcranial magnetic stimulation of the human visual cortex.

The magnocellular visual pathway is devoted to low-contrast achromatic and motion perception whereas the parvocellular pathway deals with chromatic and high resolution spatial vision. To specifically separate perception mediated by these pathways we have used low-contrast Gaussian filtered black-white or coloured visual stimuli. By use of transcranial magnetic stimulation (TMS) over the visual cortex inhibition of magnocellular stimuli was achieved distinctly earlier by about 40 ms compared with parvocellular information. A nonspecific inhibition of all stimuli could be seen peaking at 75-90 ms, significantly higher for magnocellular stimuli. The particular vulnerability of magnocellular stimuli to TMS is correlated with distinct physiological properties of this pathway such as faster conduction velocity and non-linear stimulus encoding.     

Detailed spatiotemporal brain mapping of chromatic vision combining high‐resolution VEP with fMRI and retinotopy

Neuroimaging studies have identified so far, several color‐sensitive visual areas in the human brain, and the temporal dynamics of these activities have been separately investigated using the visual‐evoked potentials (VEPs). In the present study, we combined electrophysiological and neuroimaging methods to determine a detailed spatiotemporal profile of chromatic VEP and to localize its neural generators. The accuracy of the present co‐registration study was obtained by combining standard fMRI data with retinotopic and motion mapping data at the individual level. We found a sequence of occipito activities more complex than that typically reported for chromatic VEPs, including feed‐forward and reentrant feedback. Results showed that chromatic human perception arises by the combined activity of at the least five parieto‐occipital areas including V1, LOC, V8/VO, and the motion‐sensitive dorsal region MT+. However, the contribution of V1 and V8/VO seems dominant because the re‐entrant activity in these areas was present more than once (twice in V8/VO and thrice in V1). This feedforward and feedback chromatic processing appears delayed compared with the luminance processing. Associating VEPs and neuroimaging measures, we showed for the first time a complex spatiotemporal pattern of activity, confirming that chromatic stimuli produce intricate interactions of many different brain dorsal and ventral areas.     

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.     

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.     

Impairments in generation of early-stage transient visual evoked potentials to magno- and parvocellular-selective stimuli in schizophrenia.

OBJECTIVE: Patients with schizophrenia demonstrate significant impairments of early visual processing, potentially implicating dysfunction of the magnocellular visual pathway. The present study evaluates transient visual evoked potential (tVEP) responses to stimuli biased toward the magnocellular (M) or parvocellular (P) systems in patients with schizophrenia vs. normal volunteers first to evaluate relative contributions of M and P systems to specific tVEP components in schizophrenia and, second, to evaluate integrity of early M and P processing in schizophrenia. METHODS: Seventy-four patients with schizophrenia and schizoaffective disorder were compared with 59 control subjects using separate stimuli to assess the tVEP response to M, P and mixed M/P conditions. Stimuli were biased toward M vs. P processing by manipulation of chromatic and achromatic contrast. C1, P1, N1 and P2 components were compared between patients and controls. All subjects showed 20/32 vision or better. RESULTS: Waveforms were obtained to low contrast (M), chromatic contrast (P) and high contrast (mixed M/P) stimuli in both patients and controls. C1 was present to P and mixed M/P stimuli. Patients showed a significant reduction in amplitude and an increase in latency of the C1 component. P1 was elicited primarily by M and mixed M/P stimuli, whereas N1 was elicited primarily by P and mixed M/P stimuli. Patients showed reductions in both P1 and N1 amplitudes across conditions. However, only reductions in P1 amplitude survived covariation for between group differences in visual acuity. Further, P1 amplitude reductions in the M condition correlated with a proxy measure of global outcome. CONCLUSIONS: M- and P-selective stimuli elicit differential components of the tVEP. Patients with schizophrenia show significant reductions in response even to simple visual stimuli. Deficits, particularly within the M system, may correlate significantly with global outcome and level of community functioning. SIGNIFICANCE: Whereas deficits in high-order cognitive processing have been extensively documented in schizophrenia, integrity of early-stage sensory processing has been studied to a lesser degree. The present findings suggest that deficits in early-stage visual processing are significantly related to overall clinical outcome in schizophrenia. Further, between-group differences in visual acuity may influence VEP results, even for subjects with 'normal' vision (20/32 or better).     

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.     

Coherent versus Component Motion Perception in Autism Spectrum Disorder

Research on visual perception in Autism Spectrum Disorder (ASD) tries to reveal the underlying mechanisms of aberrant local and global processing. Global motion perception is one way to study this aspect of ASD. We used plaid motion stimuli, which can be perceived as a coherently moving pattern, requiring feature integration, or as two transparent gratings sliding over each other. If global motion detection is impaired in ASD, this would lead to a decrease of the total time that a coherent pattern is perceived. However, in contrast to other studies in the literature, our results gave no evidence of impaired global motion perception in people with ASD. A reconciliation of the different outcomes is proposed based on spatial frequency processing in ASD.     

Assessment of visual functions following prenatal exposure to organic solvents

Prenatal exposure to organic solvents has been previously associated with increased risk of color vision deficits and reduced visual acuity in young children. These findings prompted us to evaluate visual functioning in solvent-exposed infants using more sensitive non-invasive visual evoked potential (VEP) techniques. VEP techniques are described in the context of an ongoing prospective longitudinal cohort study of infants exposed to organic solvents in utero. VEPs are recorded via three active electrodes fitted over the occipital cortex while infants view changing visual stimuli. The sweep VEP is used to assess contrast detection and visual acuity by presenting sinusoidal gratings that "sweep" across a range of contrasts and spatial frequencies. Transient VEPs are used to assess responses to equiluminant chromatic- and luminance-modulated sinusoidal gratings presented in pattern onset-offset format. A single case study is presented showing abnormal chromatic responses and reduced contrast sensitivity in a 2.5-year-old boy following prenatal exposure to perchloroethylene (PCE). These VEP techniques therefore appear promising for the clinical assessment of visual toxicity in pediatric populations.     

Coherent motion processing in autism spectrum disorder (ASD): An fMRI study

A deficit in global motion processing caused by a specific dysfunction of the visual dorsal pathway has been suggested to underlie perceptual abnormalities in subjects with autism spectrum disorders (ASD). However, the neural mechanisms associated with abnormal motion processing in ASD remain poorly understood. We investigated brain responses related to the detection of coherent and random motion in 15 male subjects with ASD and 15 age- and IQ-matched healthy controls (aged 13-19 years) using event-related functional magnetic resonance imaging (fMRI). Behaviorally, no significant group differences were observed between subjects with ASD and controls. Neurally, subjects with ASD showed increased brain activation in the left primary visual cortex across all conditions compared with controls. A significant interaction effect between group and condition was observed in the right superior parietal cortex resulting from increased neural activity in the coherent compared with the random motion conditions only in the control group. In addition, neural activity in area V5 was not differentially modulated by specific motion conditions in subjects with ASD. Functional connectivity analyses revealed positive correlations between the primary visual cortex and area V5 within both hemispheres, but no significant between-group differences in functional connectivity patterns along the dorsal stream. The data suggest that motion processing in ASD results in deviant activations in both the lower and higher processing stages of the dorsal pathway. This might reflect differences in the perception of visual stimuli in ASD, which possibly result in impaired integration of motion signals.     

Interhemispheric functional synchronization at the first step of visual information processing in humans.

OBJECTIVE: We examined the interhemispheric functional synchronization of the visual cortex using coherence (Coh) analysis. METHODS: Achromatic or isoluminant chromatic sinusoidal grating stimuli were presented to each hemifield at a rate of 8 reversals/s to record steady-state visual-evoked potentials (S-VEPs) in 10 healthy subjects. Four recording electrodes were placed at O1, O2, P3 and P4, referred to an electrode at Cz. A total of 50 responses of 1 s epoch were averaged, and were subjected to discrete fast Fourier transforms to yield the amplitude and phase of the 8 Hz component. Ordinary and partial Coh values were also calculated. RESULTS: For both achromatic and chromatic stimuli, the 8 Hz amplitudes of O1 and O2 were significantly larger than those of P3 and P4 without any significant difference between O1 and O2. The phase lag between O1 and O2 was approximately 30 degrees (latency shift 10.4 ms). Partial Coh between O1 and O2 at 8 Hz was significantly greater than that of the unstimulated condition, and this was only observed at 8 Hz. CONCLUSIONS: These results suggest that interhemispheric synchronization in the occipital area occurs despite the nature of the visual stimuli. Therefore, the activation of interhemispheric connection is important for the early stage of the visual information processing. SIGNIFICANCE: Our results indicate that the first step of the visual information processing requires interhemispheric functional synchronization.     

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.     

Normative data for onset VEPs to red-green and blue-yellow chromatic contrast.

OBJECTIVE: To better characterize the properties of chromatic VEPs to onset-offset of red-green and blue-yellow equiluminant patterns, and establish normative values for a set of stimuli able to elicit robust and reliable responses, suitable for the clinical application. METHODS: Chromatic VEPs have been recorded (Oz lead) from 28 normal subjects (age range 20-53 years) in response to monocular presentation of both red-green and blue-yellow equiluminant sinusoidal gratings. Stimuli were generated by a Cambridge VSG/2 card and displayed on a Barco CCID monitor (14x14 deg field size). Spatial frequency, chromaticity, contrast and onset-offset duration were varied. RESULTS: For both red-green and blue-yellow equiluminant stimuli, robust responses have been obtained with gratings of 2 c/deg, presented in onset (300 ms) offset (700 ms) mode, at contrasts ranging from 90 to 6%. In all observers, the VEP waveform consisted mainly of a negative wave at stimulus onset, with a latency rapidly increasing with decreasing contrast. For both red-green and blue-yellow stimuli, the VEP contrast threshold coincided with the psychophysical threshold. CONCLUSIONS: The results complement previous studies aimed at characterizing the properties of chromatic VEPs. In addition, normative data are provided for a set of stimulus characteristics suitable for the clinical routine.     

The contribution of human cortical area V3A to the perception of chromatic motion: a transcranial magnetic stimulation study

Area V3A was identified in five human subjects on both a functional and retinotopic basis using functional magnetic resonance imaging techniques. V3A, along with other visual areas responsive to motion, was then targeted for disruption by repetitive transcranial magnetic stimulation (rTMS) whilst the participants performed a delayed speed matching task. The stimuli used for this task included chromatic, isoluminant motion stimuli that activated either the L?M or S?(L+M) cone‐opponent mechanisms, in addition to moving stimuli that contained only luminance contrast (L+M). The speed matching task was performed for chromatic and luminance stimuli that moved at slow (2°/s) or faster (8°/s) speeds. The application of rTMS to area V3A produced a perceived slowing of all chromatic and luminance stimuli at both slow and fast speeds. Similar deficits were found when rTMS was applied to V5/MT+. No deficits in performance were found when areas V3B and V3d were targeted by rTMS. These results provide evidence of a causal link between neural activity in human area V3A and the perception of chromatic isoluminant motion. They establish area V3A, alongside V5/MT+, as a key area in a cortical network that underpins the analysis of not only luminance but also chromatically‐defined motion.     

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.     

Electrophysiological evidence for selective impairment of optic flow perception in autism spectrum disorder

People with autism spectrum disorder (ASD) often show inferior global motion performance with superior performance in detail form perception, suggesting dysfunction of the dorsal visual stream. To elucidate the neural basis of impaired global motion perception in ASD, we measured psychophysical threshold and visual event-related potentials (ERPs) with a 128-channel system in 12 ASD and 12 healthy control adults. Radial optic flow (OF) and horizontal motion (HO) were used as the visual stimuli. The former was related to the ventro-dorsal stream formed by the inferior parietal lobule, while the latter was conveyed from the dorso-dorsal stream formed by the superior parietal lobule. No significant group differences were observed in the motion thresholds for both OF and HO. N170 and P200 were elicited as major components of ERPs in both groups. However, the latencies of both components for OF but not HO were significantly prolonged in ASD compared with the control group. Our ERP results suggest that ASD has a selective impairment for OF processing even though the psychophysical thresholds are preserved. Therefore, we provide the first electrophysiological evidence for altered function of the higher-level dorsal visual stream in ASD, specifically the ventro-dorsal stream closely related to OF perception.