University of California, San Diego
The Department of Psychology is Honored
to Present a Talk by
Don MacLeod
University of California, San Diego
"Neural Limitations in Normal and Recovered Vision"
Presented on February 24, 2005
Location: The Crick Conference Room
Mandler Hall, room 3545
- Abstract:
- Why is our visual resolution limited? One answer is that any
misdirection of the optical energy or the neural signals, as they
pass from any stage to the one after it, could impair our visual
resolution of fine detail. Losses at the first stage of
transmission--the optical projection from the object to the retinal
image--can be bypassed through the use of interference fringe patterns
as targets. For these targets, the resolution limit is increased
enough to indicate an important role for optical factors in limiting
the quality of corrected normal vision. Yet, vision for these targets
is still imperfect--a reflection of neural limitations. Much of the
loss can be shown to originate relatively late in neural processing.
The progressively distributed nature of neural filtering is
appropriate for good performance in the presence of neural noise.
Form deprivation from early childhood leads to permanent impairment of vision. In one recently studied person with congenital bilateral cataracts, the neural system could not do justice to the fine detail newly available through surgery in adulthood. Another, completely blind observer, Mike May, still shows profound neural losses in resolution and profound impairment in most aspects of three dimensional perception after a successful corneal transplant. Although he correctly interpreted planar figures soon after surgery, he was quite unable to use shading to derive three dimensional shape and is developing that capacity only slowly. He inhabits a visual world of two dimensional abstract shapes and colors, and experiences great difficulty in face and object recognition. He is, however, relatively good at exploiting motion cues to three dimensional arrangement. Correspondingly, preliminary brain imaging results show more activity in the MT region than in the ventral stream.
- About the Speaker:
- I try to understand the process of human vision in physiological or
mechanistic terms, using the tools of psychophysics in conjunction with
electrophysiological and anatomical data from animals. This involves
tracing the sequence of operations that occurs as information flows from
retina to brain.
One representative project asks: Why isn't vision perfect? Bypassing optical losses by using interference fringe patterns directly generated on the retina as stimuli, we have shown that considerable information about the finest details may survive in the retinal image but be lost in neural processing, and that all of this neural loss occurs later than the primary sensitivity-regulating processes of cone vision (which must therefore be strictly local--either internal to the cones or fed by single cones). Most recently and most surprisingly, we find that unresolvably fine patterns can activate primary visual cortex and there produce pattern-specific aftereffects (such as tilt aftereffects or orientation-selective losses of visual contrast sensitivity), even though the subject can not discriminate their orientation. It follows that activation of single orientation-selective neurons in visual cortex is not a sufficient condition for perception of orientation, and that our stimuli are penetrating the visual system as far as primary visual cortex (the region of cortex thought to be most critical for the perception of detail), yet fail to penetrate to conscious experience. We hope that this can be confirmed by MRI experiments using these laser stimuli.
In another line of work, the neural coding of color and luminance is being investigated, both absolutely and in its dependence on context, with attention to known physiological nonlinearities. We are trying to characterize quantitatively the nonlinearities in the neural representation of color, and relate them on the one hand to mathematically optimal solutions to the problem of representing colors with the sort of distribution that is environmentally typical, and on the other hand to color difference data through a neurally constrained form of multi-dimensional scaling.
For More Information About This Speaker:
- http://psy.ucsd.edu/~dmacleod/
Dr. MacLeod's website.
