[Show abstract][Hide abstract] ABSTRACT: To determine whether conscious perception has access to brief temporal event, we asked subjects in an odd-man out paradigm to determine which of the four Gaussian blobs was flickering asynchronously in time. We measure synchrony thresholds as a function of the base temporal frequency for spatially scaled stimuli in foveal and peripheral vision. The results are consistent with a time delay of around 67 milliseconds (ms) for foveal vision and 91 ms for peripheral vision. We conclude that conscious perception has access to only relatively long (∼67 ms) time events.
[Show abstract][Hide abstract] ABSTRACT: Although eye dominance assessment is used to assist clinical decision-making, current understanding is limited by inconsistencies across the range of available tests. A new psychophysical test of sensory eye dominance has been developed that objectively measures the relative contribution of each eye to a fused suprathreshold binocular percept.
Six standard tests and the newly developed test were used to measure motor and sensory dominance in a group of 44 binocularly normal individuals (mean age, 29.5 ± 9.10 years). The new test required observers to perform a motion coherence task under dichoptic viewing conditions, wherein a population of moving, luminance-defined signal (coherently moving) and noise (randomly moving) dots were presented separately to each eye. The observers judged the motion direction of the signal dots. Motion coherence thresholds were measured by varying the ratio of signal-to-noise dots, in a staircase procedure.
The new dichoptic motion coherence threshold test revealed a clear bimodal distribution of sensory eye dominance strength, wherein the majority of the participants (61%) showed weak dominance, but a significant minority (39%) showed strong dominance. Subsequent analysis revealed that the strong-dominance group showed greater consistency across the range of traditional eye dominance tests used.
This new quantitative dichoptic motion coherence threshold technique suggests that there are two separate sensory eye dominance strength distributions among observers with normal binocular vision: weak and strong eye dominance. This finding may provide a basis for clinical decision-making by indicating whether eye dominance is likely to be an important consideration in a particular patient.
[Show abstract][Hide abstract] ABSTRACT: In this study, we investigate how the responses of the human visual pathway to temporal frequency are modified as information transfers between the lateral geniculate nucleus (LGN) and primary visual cortex (V1) and to the extrastriate areas of the dorsal and ventral streams (V2, V3, VP, V3A, V4, and MT). We use high-field fMRI (4 T) to record simultaneously the responses of these areas across temporal frequency for chromatic stimuli (L/M-cone opponent and S-cone opponent) and stimuli of high and low achromatic contrasts. We find that: (1) the LGN has relatively low-pass responses for temporal frequency at both high and low achromatic contrasts, indicating that LGN cell spiking activity is not well reflected in the BOLD response. In addition, M cell-like temporal responses were not found, even at low contrasts. (2) Responses in V1 and extrastriate areas V2, V3, VP, and V3A display a progressively low-pass dependence on temporal frequency for achromatic stimuli (2-16 Hz) and are flat for chromatic stimuli (2-8 Hz), showing little overall difference between chromatic and achromatic cortical temporal filtering. (3) Strongly differential effects are found between dorsal and ventral stream processing by the level of MT and V4. V4 shows a significant low-pass temporal dependence for all achromatic and chromatic stimuli, whereas MT has temporally high-pass or flat responses for achromatic and chromatic stimuli. MT was the only visual area that showed M cell-like responses. We conclude that the dorsal and ventral pathways of human vision progressively develop characteristic differences in temporal processing that affect both chromatic and achromatic stimuli.
Journal of Vision 11/2010; 10(13):13. · 2.48 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We developed a binocular treatment for amblyopia based on antisuppression therapy.
A novel procedure is outlined for measuring the extent to which the fixing eye suppresses the fellow amblyopic eye. We hypothesize that suppression renders a structurally binocular system, functionally monocular.
We demonstrate using three strabismic amblyopes that information can be combined normally between their eyes under viewing conditions where suppression is reduced. Also, we show that prolonged periods of viewing (under the artificial conditions of stimuli of different contrast in each eye) during which information from the two eyes is combined leads to a strengthening of binocular vision in such cases and eventual combination of binocular information under natural viewing conditions (stimuli of the same contrast in each eye). Concomitant improvement in monocular acuity of the amblyopic eye occurs with this reduction in suppression and strengthening of binocular fusion. Furthermore, in each of the three cases, stereoscopic function is established.
This provides the basis for a new treatment of amblyopia, one that is purely binocular and aimed at reducing suppression as a first step.
Optometry and vision science: official publication of the American Academy of Optometry 09/2010; 87(9):697-704. · 1.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Although there is general agreement that the fMRI cortical response is reduced in humans with amblyopia, the deficit is subtle and has little correlation with threshold-based psychophysics. From a purely contrast sensitivity perspective, one would expect fMRI responses to be selectively reduced for stimuli of low contrasts. However, to date, all fMRI stimuli used in studies of amblyopia have been of high contrast. Furthermore, if the deficit is selective for low contrasts, one would expect it to reflect a selective M-cell loss, because M-cells have much higher contrast gain than P-cells and make a larger contribution to the threshold detection of stimuli of low spatial and medium temporal frequencies. To test these two predictions, we compared % BOLD response between the eyes of normals and amblyopes for low- and high-contrast stimuli using a phase-encoded design. The results suggest that the fMRI deficit in amblyopia depends upon stimulus contrast and that it is greater at high contrasts. This is consistent with a selective P-cell contrast deficit at a precortical or early cortical site.
Human Brain Mapping 08/2010; 31(8):1233-48. · 6.88 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We extended a previous study (Hess et al. (1999). A deficit in strabismic amblyopia for global shape detection. Vision Research, 39, 901-914) where it was claimed that strabismic amblyopes exhibit a deficit for the detection of continuous radial frequency patterns, a task that is thought to involve global processing and in particular, a contribution of extra-striate area V4. We confirm this previous report using a novel Gabor-sampled stimulus and show that the deficit for the amblyopic eye occurs across a range of circular contour frequencies, that is the number of radial cycles per circular contour length in degrees. By arranging the Gabor-samples to coincide with either the peaks/troughs or zero-crossings of the radial modulation, we were able to tease apart the relative contributions of local position and local orientation respectively to the shape processing deficit. The deficit for the amblyopic eye involves both anomalous position and orientation coding with the latter being more affected than the former. While this suggests that ventral extra-striate processing is anomalous, it leaves open the possibility that the striate input may be responsible.
Vision research 07/2010; 50(16):1612-7. · 2.29 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigated sensitivity to motion gradients psychophysically using a band-pass filtered white noise stimulus with two superimposed components moving in opposite directions and spatially modulated with out of phase periodic functions. An optimum sensitivity ratio of the carrier to the modulator frequency of about 11 was measured. Tuning for speed was also observed, with sensitivity falling off at higher speeds in a trend showing scale invariance, consistent with temporal frequency tuning. Similar tuning properties were observed for both luminance and motion contrast thresholds. These findings are consistent with local and global processes in striate and extra-striate cortex respectively and suggest the scale of second stage low frequency integration is broad and matched to the spatiotemporal scale of the sensitivity of first stage local filters. The finding of scale invariance over a large range in stimulus size of 4.6-37 degrees of visual angle suggest a general property of integrating neural mechanisms, which was identified here because of the use of narrowband stimuli.
Vision research 05/2010; 50(15):1475-85. · 2.29 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Amblyopia or lazy eye is the most common cause of uniocular blindness in adults and is caused by a disruption to normal visual development as a consequence of unmatched inputs from the two eyes in early life, arising from a turned eye (strabismus), unequal refractive error (anisometropia), or form deprivation (e.g., cataract). Using high-field functional magnetic resonance imaging in a group of human adults with amblyopia, we previously demonstrated that reduced responses are observable at a thalamic level, that of the lateral geniculate nucleus (LGN). Here we investigate the selectivity of this deficit by using chromatic and achromatic stimuli that are designed to bias stimulation to one or other of the three ascending pathways (the parvocellular, magnocellular, and koniocellular). We find the greatest LGN deficit is for stimuli modulated along the chromatic, L/M cone opponent axis of color space, suggesting a selective loss of parvocellular function in the LGN. We also demonstrate a cortical deficit that involves all the visual areas studied (V1, V2, V3, VP, V3A, V4), and we find this is greatest for the two chromatic responses (S cone opponent and L/M cone opponent) versus the achromatic response, as might be expected from a loss of segregation of chromatic pathways in the cortex.
Journal of Neurophysiology 05/2010; 104(1):475-83. · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Much of the debate surrounding the precise functional role of brain mechanisms implicated in the processing of human faces can be explained when considering that studies into early-stage neural representations of the spatial arrangement of facial features are potentially contaminated by "higher-level" cognitive attributes associated with human faces. One way to bypass such attributes would be to employ ambiguous stimuli that are not biased toward any particular object class and analyze neural activity in response to those stimuli in a manner similar to traditional reverse correlation for mapping visual receptive fields. Accordingly, we sought to derive whole face representations directly from neural activity in the human brain using electroencephalography (EEG). We presented ambiguous fractal noise stimuli to human participants and asked them to rate each stimulus along a "face not present" to "face present" continuum while simultaneously recording EEGs. All EEGs were subjected to a time-frequency analysis near 170 ms (negative amplitudes near 170 ms post-stimulus onset have been linked to early face processing) for five different frequency bands (delta, theta, alpha, beta, and gamma) on a trial-by-trial basis, independent of the behavioral responses. Images containing apparent face-like structure were obtained for theta through gamma frequency bands for strong negative amplitudes near 170 ms post-stimulus onset. The presence of the face-like structure in the spatial images derived from brain signals was objectively verified using both Fourier methods and trained neural networks. The results support the use of a modified reverse correlation technique with EEG as a non-biased assessment of brain processes involved in the complex integration of spatial information into objects such as human faces.
[Show abstract][Hide abstract] ABSTRACT: The present treatments for amblyopia are predominantly monocular aiming to improve the vision in the amblyopic eye through either patching of the fellow fixing eye or visual training of the amblyopic eye. This approach is problematic, not least of which because it rarely results in establishment of binocular function. Recently it has shown that amblyopes possess binocular cortical mechanisms for both threshold and suprathreshold stimuli.
We outline a novel procedure for measuring the extent to which the fixing eye suppresses the fellow amblyopic eye, rendering what is a structurally binocular system, functionally monocular.
Here we show that prolonged periods of viewing (under the artificial conditions of stimuli of different contrast in each eye) during which information from the two eyes is combined leads to a strengthening of binocular vision in strabismic amblyopes and eventual combination of binocular information under natural viewing conditions (stimuli of the same contrast in each eye). Concomitant improvement in monocular acuity of the amblyopic eye occurs with this reduction in suppression and strengthening of binocular fusion. Furthermore, in a majority of patients tested, stereoscopic function is established.
This provides the basis for a new treatment of amblyopia, one that is purely binocular and aimed at reducing suppression as a first step.
Restorative neurology and neuroscience 01/2010; 28(6):793-802. · 2.93 Impact Factor