Uma Shahani

Illusory percepts of moving patterns due to discrete temporal sampling

Continuously, moving objects under continuous illumination can be seen to move in a direction opposite to their actual motion. This illusory reversed motion can be explained as due to discrete temporal sampling of the moving stimulus by the visual system. If temporal sampling lies behind the illusory motion, then the probability of illusory motion should depend on the temporal frequency of the motion stimulus. By presenting contracting bulls-eye gratings of various spatial frequencies we were able to tease apart the drift speed and temporal frequency. The prevalence of illusory percepts depended on the temporal frequency, not the speed. The data suggest that the human visual system samples the incoming stimulation at a rate near 16 Hz.

Absolute quantification of oxygenated hemoglobin within the visual cortex with functional near infrared spectroscopy (fNIRS).

PURPOSE To examine absolute changes in oxygenated (HbO) and deoxygenated (Hb) hemoglobin concentrations over the visual cortex in response to visual stimulation. Before this study, only relative changes have been reported at the visual cortex. METHODS Near infrared spectroscopy (NIRS) can be used to assess changes in hemoglobin concentration in tissue. A frequency domain oximeter with a specially designed probe was placed over the visual cortex while the participant viewed a checkerboard stimulus. The stimulus was alternated at 7.5 Hz for 30 seconds before being replaced by a control gray screen. RESULTS The mean HbO concentration when the stimulus was presented was 26.8+/-3.9 microM, as opposed to 25.9+/-3.9 microM for the control condition, and the mean Hb concentration was 18.8+/-2.3 microM during stimulation and 19.1+/-2.3 microM during the control condition. The greatest change in HbO concentration occurred within the first 10 seconds. It did not increase significantly (P>0.01) after that time. CONCLUSIONS This study is the first to demonstrate absolute quantification of HbO and Hb concentrations in the visual cortex with functional changes in hemoglobin concentrations in response to a visual stimulus. NIRS has the potential to be a valuable clinical tool in assessing the hemodynamics of the visual system in a quantitative and localized manner.

Localization of hemodynamic responses to simple visual stimulation: An fNIRS study

PURPOSE The purpose of the study was to use functional near-infrared spectroscopy (fNIRS) to explore the extent of activation in occipito-parietal cortices to high-contrast checkerboard stimuli. The distributions of oxyhemoglobin (HbO), deoxyhemoglobin (Hb), and total hemoglobin (THb) concentrations were used as measures of cortical activation. METHODS Data were collected sequentially using the Frequency Domain Multi-Distance oximeter to record absolute chromophore concentration. Responses to three presentation modes (static, pattern reversal, and ON/OFF stimulation) were compared over 15 locations in two participants. The most effective stimulus was used in 10 participants at the most responsive occipito-parietal sites. RESULTS Pattern-reversal stimulation evoked the largest increase in HbO, and this increase was greatest at O1 and O2 (5% to the right and left of the midline occipital location Oz) and diminished at recording locations over the posterior parietal regions in the vertical direction. Hb changes were smaller than those observed for HbO. Significantly smaller responses were recorded over the midline (Oz) compared with those at O1 and O2. Changes in hemoglobin concentration reflected the location of activated brain tissue. CONCLUSIONS The authors have demonstrated the distribution of the hemodynamic response using absolute values of hemoglobin chromophores in response to simple but strong stimulation using checkerboard presentations.

Reduced Haemodynamic Response in the Ageing Visual Cortex Measured by Absolute fNIRS.

The effect of healthy ageing on visual cortical activation is still to be fully explored. This study aimed to elucidate whether the haemodynamic response (HDR) of the visual cortex altered as a result of ageing. Visually normal (healthy) participants were presented with a simple visual stimulus (reversing checkerboard). Full optometric screening was implemented to identify two age groups: younger adults (n = 12, mean age 21) and older adults (n = 13, mean age 71). Frequency-domain Multi-distance (FD-MD) functional Near-Infrared Spectroscopy (fNIRS) was used to measure absolute changes in oxygenated [HbO] and deoxygenated [HbR] haemoglobin concentrations in the occipital cortices. Utilising a slow event-related design, subjects viewed a full field reversing checkerboard with contrast and check size manipulations (15 and 30 minutes of arc, 50% and 100% contrast). Both groups showed the characteristic response of increased [HbO] and decreased [HbR] during stimulus presentation. However, older adults produced a more varied HDR and often had comparable levels of [HbO] and [HbR] during both stimulus presentation and baseline resting state. Younger adults had significantly greater concentrations of both [HbO] and [HbR] in every investigation regardless of the type of stimulus displayed (p<0.05). The average variance associated with this age-related effect for [HbO] was 88% and [HbR] 91%. Passive viewing of a visual stimulus, without any cognitive input, showed a marked age-related decline in the cortical HDR. Moreover, regardless of stimulus parameters such as check size, the HDR was characterised by age. In concurrence with present neuroimaging literature, we conclude that the visual HDR decreases as healthy ageing proceeds.

Neural and Vascular Responses to Fused Binocular Stimuli:A VEP and fNIRS Study

PURPOSE The aim of our study was to investigate the correlation between neural and hemodynamic responses to stereoscopic stimuli recorded over visual cortex. METHODS Test stimuli consisted of a static checkerboard (checks) and dichoptic static random dot (RD) presentations with no binocular disparity (ZD) or with horizontal disparity (HD). Hemodynamic responses were recorded from right and left occipital sites using functional near-infrared spectroscopy (fNIRS). Visual evoked potentials (VEPs) were recorded over three occipital sites to the onset of the same stimuli. RESULTS Early components, N1 and P2, were sensitive to HD, suggesting that an enhanced N1-reduced P2 complex could be an indicator of binocular disparity and stereopsis. VEPs to checks and ZD stimulation were similar. fNIRS recordings showed changes in hemodynamic activation from baseline levels in response to all stimuli. In general, HD elicited a larger vascular response than ZD. Oxyhemoglobin concentration (HbO) was correlated with the VEP amplitude during the checks and HD presentations. CONCLUSIONS We report an association between neural and hemodynamic activation in response to checks and HD. In addition, the results suggested that N1-P2 complex in the VEP could be a neural marker for stereopsis and fNIRS demonstrated differences in HbO. Specifically, checks and HD elicited larger hemodynamic responses than random dot patterns without binocular disparity.

Two eyes: 2 better than one?

Classical data on the detection of simple patterns show that two eyes are more sensitive than one eye. The degree of binocular summation is important for inferences about the underlying combination mechanism. In a signal detection theory framework, sensitivity is limited by internal noise. If noise is added centrally after binocular combination, binocular sensitivity is expected to be twice as good as monocular. If the noise is added peripherally at each eye prior to combination, binocular sensitivity will be sqrt[2] higher than monocular. In a large sample of observers (51), we measured contrast sensitivity for detection of gratings at several spatial frequencies using left, right, or both eyes. Estimates of binocular summation using both binocular summation ratios and Minkowski coefficients show a summation ratio with means in the range of 1.5-1.6. The 95% confidence interval overlaps with the value of sqrt[2] predicted by the peripheral noise model and does not overlap with the value of 2 predicted by the central noise model.

Investigating the mechanisms that may underlie the reduction in contrast sensitivity during dynamic accommodation

Head and eye movements, together with ocular accommodation enable us to explore our visual environment. The stability of this environment is maintained during saccadic and vergence eye movements due to reduced contrast sensitivity to low spatial frequency information. Our recent work has revealed a new type of selective reduction of contrast sensitivity to high spatial frequency patterns during the fast phase of dynamic accommodation responses compared with steady-state accommodation. Here were report data which show a strong correlation between the effects of reduced contrast sensitivity during dynamic accommodation and velocity of accommodation responses, elicited by ramp changes in accommodative demand. The results were accounted for by a contrast gain control model of a cortical mechanism for contrast detection during dynamic ocular accommodation. Sensitivity, however, was not altered during attempted accommodation responses in the absence of crystalline-lens changes due to cycloplegia. These findings suggest that contrast sensitivity reduction during dynamic accommodation may be a consequence of cortical inhibition driven by proprioceptive-like signals originating within the ciliary muscle, rather than by corollary discharge signals elicited simultaneously with the motor command to the ciliary muscle.

Haemodynamic Responses to Radial Motion in the Visual Cortex

Functional near-infrared spectroscopy (fNIRS) is an optical imaging technique that relies on emitting near-infrared light into cortical tissue to measure changes in haemoglobin concentrations as a result of stimulation. The purpose of this study was to observe haemodynamic changes in response to moving stimuli over the primary visual cortex. The test stimuli were radially expanding and contracting concentric gratings and the control stimulus was a matched stationary pattern. A two-channel oximeter (Oxiplex TS) was used to record changes in oxyhaemoglobin (HbO), de-oxyhaemoglobin (Hb) and total haemoglobin concentrations (THb). An increase in haemodynamic activation was observed 20s after the onset of motion and maintained for up to 20s following motion offset (30s time window). This was compared to a time window of 10s before the offset of the stationary presentation. Differences between expanding and contracting motion did not achieve significance at any of the locations. However, a significantly larger HbO response was observed across the 30s time window when compared to the baseline window at both occipital locations. Preliminary results from more temporal locations also showed a similar trend. It is suggested that the delayed rise in HbO levels could be due to an inconsistent maintenance of luminance and contrast-related features of the moving stimuli in V1 receptive fields. Furthermore, motion-after effects could have contributed to delaying the drop to baseline levels. The current study has shown some evidence that fNIRS can be used to record haemodynamic responses to moving stimuli from the visual cortex. We suggest that longer durations of randomised moving, stationary and grey screen presentations would be useful in teasing apart pure motion responses and after-effects. fNIRS could also potentially be used to investigate haemodynamic changes in disorders such as amblyopia that present with motion processing deficits.

Magnetoencephalography and stereopsis: rhythmic cortical activity in humans recorded over the parieto-occipital cortex.

We have studied human stereopsis by analysing magnetoencephalographic signals during the presentation of stereograms using frequency analysis. The study of synchronised firing of cortical neurones is a new way of understanding information processing in the brain and it is hypothesised that frequencies greater than 35 Hz are used for higher-order processing. We report the response of cortical neurones involved in stereopsis recorded from over the occipital and parietal cortices using a single channel axial superconducting quantum interference device neuromagnetometer. Our main result was increased cortical activity in the gamma-band at frequencies apparently related to stereopsis and the perception of depth. Our results are consistent with reports in the literature that suggest that frequencies above 40 Hz are involved in attention, pattern recognition and higher order visual activity.

Age-Related Changes in Global Motion Coherence: Conflicting Haemodynamic and Perceptual Responses

Our aim was to use both behavioural and neuroimaging data to identify indicators of perceptual decline in motion processing. We employed a global motion coherence task and functional Near Infrared Spectroscopy (fNIRS). Healthy adults (n = 72, 18–85) were recruited into the following groups: young (n = 28, mean age = 28), middle-aged (n = 22, mean age = 50), and older adults (n = 23, mean age = 70). Participants were assessed on their motion coherence thresholds at 3 different speeds using a psychophysical design. As expected, we report age group differences in motion processing as demonstrated by higher motion coherence thresholds in older adults. Crucially, we add correlational data showing that global motion perception declines linearly as a function of age. The associated fNIRS recordings provide a clear physiological correlate of global motion perception. The crux of this study lies in the robust linear correlation between age and haemodynamic response for both measures of oxygenation. We hypothesise that there is an increase in neural recruitment, necessitating an increase in metabolic need and blood flow, which presents as a higher oxygenated haemoglobin response. We report age-related changes in motion perception with poorer behavioural performance (high motion coherence thresholds) associated with an increased haemodynamic response.