Retinotopy of emotion: Perception of negatively valenced stimuli presented at different spatial locations as revealed by event‐related potentials

Scarce previous data on how the location where an emotional stimulus appears in the visual scene modulates its perception suggest that, for functional reasons, a perceptual advantage may exist, vertically, for stimuli presented at the lower visual field (LoVF) and, horizontally, for stimuli presented at the left visual field (LeVF). However, this issue has been explored through a limited number of spatial locations, usually in a single spatial dimension (e.g., horizontal) and invariant eccentricities. Event‐related potentials (ERPs) were recorded from 39 participants perceiving brief neutral (wheels) and emotional stimuli (spiders) presented at 17 different locations, one foveal and 16 at different peripheral coordinates. As a secondary scope, we explored the role of the magnocellular (M) and the parvocellular (P) visual pathways by presenting an isoluminant/heterochromatic (P‐biased) and a heteroluminant/isochromatic version (M‐biased) of each stimulus. Emo > Neu effects were observed in PN1 (120 ms) for stimuli located at fovea, and in PN2 (215 ms) for stimuli located both at fovea and diverse peripheral regions. A factorial approach to these effects further revealed that: (a) emotional stimuli presented in the periphery are efficiently perceived, without evident decrease from para‐ to perifovea; (b) peripheral Emo > Neu effects are reflected 95 ms later than foveal Emo > Neu effects in ERPs; (c) LoVF is more involved than UVF in these effects; (d) our data fail to support the LeVF advantage previously reported, and (e) Emo > Neu effects were significant for both M and P stimuli.     

Dysfunction of early-stage visual processing in schizophrenia.

OBJECTIVE: Schizophrenia is associated with deficits in higher-order processing of visual information. This study evaluated the integrity of early visual processing in order to evaluate the overall pattern of visual dysfunction in schizophrenia. METHOD: Steady-state visual-evoked potential responses were recorded over the occipital cortex in patients with schizophrenia and in age- and sex-matched comparison volunteers. Visual-evoked potentials were obtained for stimuli composed of isolated squares that were modulated sinusoidally in luminance contrast, number of squares, or chromatic contrast in order to emphasize magnocellular or parvocellular visual pathway activity. RESULTS: Responses of patients to magnocellular-biased stimuli were significantly lower than those of comparison volunteers. These lower response levels were observed in conditions using both low luminance contrast and large squares that biased processing toward the magnocellular pathway. In contrast, responses to stimuli that biased processing toward the parvocellular pathway were not significantly different between schizophrenia patients and comparison volunteers. A significant interaction of group and stimulus type was observed in the condition using low luminance contrast. CONCLUSIONS: These findings suggest a dysfunction of lower-level visual pathways, which was more prominent for magnocellular than parvocellular biased stimuli. The magnocellular pathway helps in orienting toward salient stimuli. A magnocellular pathway deficit could contribute to higher-level visual cognitive deficits in schizophrenia.     

Impaired critical flicker frequency in recovered optic neuritis

Thirteen patients with optic neuritis who recovered normal visual acuity and kinetic perimetry all showed impairment of foveal critical flicker frequency, a psychophysical test of visual temporal resolution. Because the magnocellular visual system demonstrates higher temporal resolution than the parvocellular system, this finding implicates involvement of axonal projections of magnocellular retinal ganglion cells in recovered optic neuritis.     

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.     

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.     

Magnocellular contributions to impaired motion processing in schizophrenia

Patients with schizophrenia show impairments in motion processing, along with deficits in lower level processing primarily involving the magnocellular visual pathway. The present study investigates potential magnocellular contributions to impaired motion processing in schizophrenia using a combined neurophysiological and behavioral approach. As compared to prior motion studies in schizophrenia, thresholds were determined for both incoherent and coherent visual motion. In this study, velocity discrimination thresholds were measured for schizophrenia patients (n=14) and age-matched normal control subjects (n=16) using a staircase procedure. Early visual processing was evaluated using steady-state visual evoked potentials (ssVEP), with stimuli biased toward activation of either the magnocellular or parvocellular visual pathways through luminance contrast manipulation. Patients with schizophrenia showed poor velocity discrimination for both incoherent and coherent motion, with no significant group x task interaction. Further, when coherent motion performance was measured at individually determined incoherent motion thresholds, accuracy levels for patients were similar to controls, also indicating similarity of deficit for incoherent vs. coherent motion discrimination. Impairments in velocity discrimination correlated significantly with reduced amplitude of ssVEP elicited by magnocellular -- but not parvocellular -- selective stimuli. This study demonstrates that deficits in motion processing in schizophrenia are significantly related to reduced activation of the magnocellular visual system. Further, this study supports and extends prior reports of impaired motion processing in schizophrenia, and indicates significant bottom-up contributions to higher-order cognitive impairments.     

Deficits in visual feature binding under isoluminant conditions

The contribution of the magnocellular stream to visual feature binding was examined psychophysically through the use of isoluminant stimuli. Subjects were presented with three briefly flashed colored letters arranged in an array and asked to identify the shape and color of the center letter. The rate of illusory conjunctions was much higher when the letters were isoluminant with a gray background, compared to when the letters were either brighter or dimmer. Over 90% of conjunction errors involved pairing the wrong shape with the correct color, rather than vice versa. Directing attention to the target location with a nonisoluminant cue did not reduce illusory conjunctions. High rates of binding errors under isoluminance are interpreted here in terms of abnormalities in visual form processing rather than an attentional effect. In another experiment designed to examine the role of synchrony in feature binding, the rate of illusory conjunctions was highest when flanking letters were presented before the central target letter and not synchronously.     

Magnocellular and parvocellular contributions to backward masking dysfunction in schizophrenia

Patients with schizophrenia have repeatedly shown deficits in visual processing. These deficits have been well documented using visual backward masking (VBM). The VBM deficit in schizophrenia is thought to be due to aberrant interactions between magnocellular (M) and parvocellular (P) visual pathways. To date, no study has studied these claims with rigorous stimuli isolating M and P pathway responses. This study examined the function of each pathway and their interactions by creating M- and P-biased targets based on their known physiological properties. The M system responds to very low luminance contrast whereas the P system does not, and the P system responds to color contrast whereas the M system generally does not. Thus, to activate the P system, target letters and masks utilized color contrast, and to activate the M system, target letters and masks utilized very low luminance contrast. Four conditions were presented such that M- and P-biased targets were paired with both M- and P-biased masks. A significant Group x Mask Condition interaction was found when a P target was used in combination with an M or P mask, but not when an M target was used. In particular, schizophrenia patients needed significantly longer interstimulus intervals (ISIs) than controls to escape from masking in the P target/M mask condition, but not in any of the other three conditions. In addition, the critical stimulus durations (CSDs) for unmasked stimuli were significantly increased for both M and P targets in patients relative to controls.These findings demonstrate a significant impairment in M, but not P pathway, function in patients with schizophrenia. Furthermore, deficits of letter identification, including those of P targets, may also reflect impairment of the M pathway given the priming function of the dorsal stream.     

Attention to central and peripheral visual space in a movement detection task. III. Separate effects of auditory deprivation and acquisition of a visual language

We employed event-related brain potentials (ERPs) and measures of signal detectability to compare attention to peripheral and central visual stimuli in normal hearing subjects who were born to deaf parents (HD Ss) and whose first language was American Sign Language (ASL). The results were compared with those obtained from normal hearing Ss and congenitally deaf Ss in the same paradigm. Task performance and ERPs during attention to the foveal region were similar in the 3 groups. In contrast, with attention to the peripheral stimuli the deaf Ss displayed attention effects over the occipital regions of both hemispheres that were several times larger than those in the hearing and the HD Ss. However, both HD and deaf Ss displayed lateral asymmetries in behavior and ERPs that were opposite in direction to those of the hearing Ss. Whereas hearing Ss detected the direction of target motion better when it occurred in the left visual field, deaf and HD Ss performed better for right visual field targets. Consistent with these results, the amplitude of the attention-related increases in the ERPs were larger from temporal and parietal regions of the right than the left hemisphere in hearing Ss, but were larger from the left than the right hemisphere in both the HD and the deaf Ss. These results suggest that auditory deprivation and the acquisition of a visual language have marked and different effects on the development of cortical specializations in humans.     

The time course of visual backward masking deficits in schizophrenia

Schizophrenia, it has been hypothesized, is linked to a deficiency in the magnocellular portion of the visual system. Abnormal backward masking has been invoked as support for this hypothesis. The rationale for linking backward masking to the magnocellular system is the hypothesis that fast responses in the magnocellular systems catches up with, and then inhibits slower responses in the parvocellular system. However, the latency difference between the magno- and parvocellular systems is at most 20 ms. Magnocellular abnormalities as a result would be expected to manifest themselves only at relatively short stimulus onset asynchronies (SOAs) or interstimulus intervals (ISIs). The present study examines this implication. It is found that a substantial number of investigations have uncovered abnormal masking at SOAs or ISIs of 300 ms or larger, and some even at ISIs as large as 700 ms. It is difficult to reconcile abnormalities at these SOAs and ISIs with magno-parvocellular latency differences of 20 ms or less. It is concluded that the abnormal masking does not support the existence of a magnocellular deficiency in schizophrenia.     

On identifying magnocellular and parvocellular responses on the basis of contrast-response functions

It has been proposed that magnocellular and parvocellular sensitivity in schizophrenic individuals can be assessed using steady-state visually evoked potentials (VEPs) to either low-contrast stimuli or stimuli whose contrast is modulated around a high contrast "pedestal" (Green MF, Butler PD, Chen Y, et al. Schizophr Bull. 2009;35:163-181). This suggestion faces 2 difficulties: (1) To use low-contrast stimuli to activate the magnocellular system is inconsistent with lesion studies that have shown that under many conditions, the parvocellular system responds to the lowest contrasts and (2) To rely on contrast-response relationships to identify magnocellular and parvocellular responses is difficult because other neurons exist in the visual system that have contrast-response relationships similar to those of magnocellular and parvocellular cells.     

Altered visual-evoked potentials in congenitally deaf adults

Visual-evoked potentials recorded from the scalp of congenitally deaf adults were significantly larger over both auditory and visual cortical areas than in normal hearing adults. Over temporal and frontal areas peripheral stimuli presented at long intervals elicited N150 components which were larger in deaf than in hearing subjects. Over occipital and parietal areas peripheral and foveal stimuli elicited larger P230 components in deaf than in hearing subjects. These results imply that early auditory experience influences the organization of the human brain for visual processing.     

Stereologic analysis of the lateral geniculate nucleus of the thalamus in normal and schizophrenic subjects

Reduction of volume and neuronal number has been found in several association nuclei of the thalamus in schizophrenic subjects. Recent evidence suggests that schizophrenic patients exhibit abnormalities in early visual processing and that many of the observed perceptual deficits are consistent with dysfunction of the magnocellular pathway, i.e. the visual relay from peripheral retinal cells to the two ventrally located magnocellular layers of the lateral geniculate nucleus (LGN). The present study was undertaken to determine whether abnormalities in cell number and volume of the LGN are associated with schizophrenia and whether the structural alterations are restricted to either the magnocellular or parvocellular subdivisions of the LGN. Series of Nissl-stained sections spanning the LGN were obtained from 15 schizophrenic and 15 normal control subjects. The optical disector/fractionator sampling method was used to estimate total neuronal number, total glial number and volume of the magnocellular and parvocellular subdivisions of the LGN. Cell number and volume of the LGN in schizophrenic subjects were not abnormal. Volume of both parvocellular and magnocellular layers of the LGN decreased with age. These findings do not support the hypothesis that early visual processing deficits in schizophrenic subjects are due to reduction of neuronal number in the LGN.     

P and M channel-specific interference in the what and where pathway

It was the purpose of this experiment to study interference effects of colour or luminance peripheral flicker (in order to saturate either the parvocellular or the magnocellular stream) on object identity and spatial location memory. Colour flicker interfered with object identity recognition, whereas luminance flicker affected memory for spatial location. Moreover, overall performance was worse if coloured rather than grey-scaled objects were used in the stimulus display. There was no selective effect of coloured flicker affecting coloured objects and chromatic flicker affecting chromatic objects. These results provide strong evidence for the theoretical position that the what pathway relies heavily on information derived from the P stream and the where pathway on the M stream.     

Magnocellular and parvocellular contributions to responses in the middle temporal visual area (MT) of the macaque monkey

Many lines of evidence suggest that the visual signals relayed through the magnocellular and parvocellular subdivisions of the primate dorsal LGN remain largely segregated through several levels of cortical processing. It has been suggested that this segregation persists through to the highest stages of the visual cortex, and that the pronounced differences between the neuronal response properties in the parietal cortex and inferotemporal cortex may be attributed to differential contributions from magnocellular and parvocellular signals. We have examined this hypothesis directly by recording the responses of cortical neurons while selectively blocking responses in the magnocellular or parvocellular layers of the LGN. Responses were recorded from single units or multiunit clusters in the middle temporal visual area (MT), which is part of the pathway leading to parietal cortex and thought to receive primarily magnocellular inputs. Responses in the MT were consistently reduced when the magnocellular subdivision of the LGN was inactivated. The reduction was almost always pronounced and often complete. In contrast, parvocellular block rarely produced striking changes in MT responses and typically had very little effect. Nevertheless, unequivocal parvocellular contributions could be demonstrated for a minority of MT responses. At a few MT sites, responses were recorded while magnocellular and parvocellular blocks were made simultaneously. Responses were essentially eliminated for all these paired blocks. These results provide direct evidence for segregation of magnocellular and parvocellular contributions in the extrastriate visual cortex and support the suggestion that these signals remain largely segregated through the highest levels of cortical processing.     

A new color vep procedure discloses asymptomatic visual impairments in optic neuritis and glaucoma suspects

OBJECTIVE: To evaluate the reliability of visual evoked potentials obtained with a set of multiple chromatic and achromatic patterns (C-VEPs) in differentiating asymptomatic perifoveal retinal impairment from central conduction impairment. METHODS: We propose a set of colored pattern stimuli that allows relatively differential activation of the magnocellular and parvocellular pathways. The system runs on a standard Pentium PC with peripherals that present stimuli and collect, analyze and print data. P1 latencies of C-VEPs obtained with achromatic (black/white) and chromatic (blue/black and red/black isocontrast) checkerboards were evaluated in normal subjects and patients with subclinical retinal impairment (glaucoma suspects) or mild neural conduction impairment (optic neuritis), none of whom had subjective visual defects. RESULTS: The procedure evoked robust cortical signals and statistically distinguished the 3 groups of subjects. The achromatic and chromatic stimuli used distinguished controls from glaucoma suspects and patients with optic neuritis. Glaucoma suspects had greater impairment of C-VEPs to blue/black checkerboards whereas patients with optic neuritis had greater impairment of responses to red/black stimuli. CONCLUSIONS: Our data suggest that chromatic patterns (color/ black, red and blue), that may activate the parvocellular and magnocellular systems differentially but not selectively, can distinguish between mild perifoveal or foveal conduction impairment. They have the additional advantage of evoking large, stable responses across all the subjects.     

A few remarks on attention and magnocellular deficits in schizophrenia

In connection with schizophrenia, it has been proposed that the magnocellular system is specifically linked to the guiding of covert visual attention. The argument is that the magnocellular pathway provides input to the dorsal cortical stream which then projects back to area V1. We review problems with this model. (1) It requires that responses in the magnocellular system have a lead time over responses in the parvocellular system. However, measurements indicate that the actual response time difference between the two systems is small or negligible when entering the visual cortex. (2) Attention can be modified by stimuli that do not activate the magnocellular system. And, (3) lesions to area MT in the dorsal stream impair smooth pursuit eye movements, but not saccadic eye movements which are associated with shifts in attention. For these reasons, it is difficult to link attention defects in schizophrenia to potential magnocellular deficits.     

Foveal and eccentric acuity in one-eyed observers.

The purpose of these two experiments was to examine the effects of enucleation on the foveal and near-periphery acuity of human observers. In Experiment 1, using high contrast illiterate-E optotypes, we measured foveal and 7-degree eccentric (nasal and temporal) acuity in the remaining eye of unilaterally enucleated children and adults. The data were compared to those of the fellow or 'good eye' of age-matched non-amblyopic and amblyopic strabismic observers-a different kind of monocular deprivation-and binocularly normal subjects. The main finding was that the 7-degree eccentric acuity of enucleated observers and normal controls was best, followed by that of the non-amblyopic and lastly, the amblyopic subjects. There were no naso-temporal asymmetries. In Experiment 2, foveal and 7-degree eccentric acuities of enucleated and binocularly normal subjects were tested at 96, 13.5 and 4.7% contrast levels using the same kind of stimuli. The foveal acuity of the enucleated observers was better than the monocular acuity of controls at all contrasts, and equivalent to their binocular acuity. The eccentric acuity of the enucleated observers was better at the two low contrast levels. No naso-temporal asymmetries were found in either group at any contrast level. These findings suggest that, in humans, visual deprivation from unilateral enucleation does not result in a naso-temporal asymmetry in the near periphery.     

Altered 'three-flash' illusion in response to two light pulses in schizophrenia

BACKGROUND: Disorganization is a core dysfunction in schizophrenia. Coherent thought and behavior rely on the interactive neural responses to temporally discrete external events. Previous studies have demonstrated that a single visual stimulus (event) is abnormally affected by another (as in backward masking), but the integration (or 'synthesis') of temporally discrete events remains largely unexplored in schizophrenia. We examined the perceived interaction of two elementary visual events in schizophrenia patients, using a psychophysical approach. METHODS: Two brief, spatially-coincident foveal light pulses (5 ms) were presented with different inter-stimulus intervals (ISIs). At ISIs around 100 ms, a substantial proportion of the light pulse pairs was paradoxically perceived as three flashes, a known phenomenon in normal subjects. The subjects reported the number of flashes perceived ('one', 'two' or 'three'). RESULTS: Schizophrenia patients (n=28) reported fewer 'three flashes' than normal controls (n=26) at the ISIs where 'three flash' reports were greatest in normal subjects (90 to 110 ms). On the other hand, at longer ISIs (130-310 ms) patients reported 'three flashes' in more trials than did normal subjects. The perception of three flashes in patients was correlated with certain aspects of clinical status, including the positive and general subscales of the PANSS. DISCUSSION: The alteration of the 'three-flash' illusion in schizophrenia suggests that the synthesis of discrete visual events is temporally 'dilated' or distorted, which might contribute to disorganized thought and behavior.     

Attention to central and peripheral visual space in a movement detection task: an event-related potential and behavioral study. I. Normal hearing adults

The effects of focussed attention to peripherally and centrally located visual stimuli were compared via an analysis of event-related brain potentials (ERPs) while subjects detected the direction of motion of a white square in a specified location. While attention to both peripheral and foveal stimuli produced enhancements of the early ERP components, the distribution over the scalp of the attention-related changes varied according to stimulus location. The attention-related increase in the amplitude of the N1 wave (157 ms) to the peripheral stimuli was greater over the parietal region of the hemisphere contralateral to the attended visual field. By contrast, the largest effects of foveally directed attention occurred over the occipital regions where the increase was bilaterally symmetrical. Additionally, the effects of attention on the ERPs were significantly larger for moving than for stationary stimuli, and this effect was greater for peripheral than for central attention. A long-latency positive displacement component (300-600 ms) was larger over the right than the left hemisphere during attention to the lateral visual fields, but was symmetrical in amplitude when central stimuli were attended. These results suggest that different pathways are modulated when attention is deployed to different regions of the visual fields. Further, they suggest that the special role of the right hemisphere in spatial attention may be limited to analysis of information in the visual periphery.