Abstract
Random dot kinematograms (RDKs) have recently been used to train subjects with cortical scotomas to perform direction of motion discrimination, partially restoring visual motion perception. To study the recovery of visual perception, it is important to understand how visual areas in normal subjects and subjects with cortical scotomas respond to RDK stimuli. Studies in normal subjects have shown that blood oxygen level-dependent (BOLD) responses in human area hV5/MT+ increase monotonically with coherence, in general agreement with electrophysiology studies in primates. However, RDK responses in prior studies were obtained while the subject was performing fixation, not a motion discrimination condition. Furthermore, BOLD responses were gauged against a baseline condition of uniform illumination or static dots, potentially decreasing the specificity of responses for the spatial integration of local motion signals (motion coherence). Here, we revisit this question starting from a baseline RDK condition of no coherence, thereby isolating the component of BOLD response due specifically to the spatial integration of local motion signals. In agreement with prior studies, we found that responses in the area hV5/MT+ of healthy subjects were monotonically increasing when subjects fixated without performing a motion discrimination task. In contrast, when subjects were performing an RDK direction of motion discrimination task, responses in the area hV5/MT+ remained flat, changing minimally, if at all, as a function of motion coherence. A similar pattern of responses was seen in the area hV5/MT+ of subjects with dense cortical scotomas performing direction of motion discrimination for RDKs presented inside the scotoma. Passive RDK presentation within the scotoma elicited no significant hV5/MT+ responses. These observations shed further light on how visual cortex responses behave as a function of motion coherence, helping to prepare the ground for future studies using these methods to study visual system recovery after injury.
Highlights
Visual motion perception enables us to navigate the environment and avoid collisions with obstacles
blood oxygen level-dependent (BOLD) signal in the area human Visual 5 area (hV5)/middle temporal (MT)+ (Figures 3B,C) showed strong statistically significant coherence dependence [F(3,11) = 6.21 p = 0.01], one-way analysis of variance (ANOVA) over coherence across subjects, each measurement reflecting the mean BOLD response amplitude averaged across trials at the same level of coherence for each subject)
Area hV5/MT+ BOLD response increased with coherence, reaching approximately 0.5% greater than baseline at 100% motion coherence
Summary
Visual motion perception enables us to navigate the environment and avoid collisions with obstacles. RDK stimuli force the visual system to extract the global coherent direction of motion from local motion signals that have to be integrated over space and time (Braddick, 1974; Newsome and Pare, 1988; Watamaniuk et al, 1993; Scase et al, 1996) before motion direction can be perceived. It is important to study the response of visual areas to RDK stimuli in healthy humans, as well as in subjects with cortical scotomas at baseline, prior to training, in order to better understand how global motion integration is processed in patients versus normal controls. It is important to study RDK processing under two different conditions, that is, when subjects perform a task related to motion discrimination versus a motion-unrelated task at fixation, as task performance is known to modulate cortical area responsiveness (Huxlin et al, 2009)
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