"A new look at the visual system" by Robert Galambos
Professor Robert Galambos is an iconic figure in Neurosciences. Along with six other faculty, he was instrumental in establishing the first academic neurosciences department in the world at UCSD. He was elected to the National Academy of Sciences in 1960. He is most widely known for his co-discovery of the phenomenon of echolocation in bats.
I had the incredible good fortune of listening to his talk at UCSD. At an age that is nearing 91, he is still amazingly sharp and full of energy. He described joint work with G. Juhasz's group. A key idea is that eye fixates on different aspects of the visual scene at rate of 3 fixations/second. Each fixation causes retina to transmit an optic nerve signal that lasts 300ms. The amazing fact is that 300ms seems to be a constant. Different stimuli, different mental states of animal, different duration of stimuli, or different magnitude of stimuli do not seem to affect this number! This was established via experiments in rats.
In humans, he described the following experiment. A red LED is flashed followed by inter stimulus interval (ISI) followed by a green LED. Four cases are observed:
- When ISI is less than 60 ms, the subject sees a single flash of orange color.
- When ISI is in range 61-90ms, the subject sees a smaller flash followed by another larger flash -- both are orange in color.
- When ISI is in range 91-290ms, the subject sees a smaller red flash followed by a larger green flash.
- When ISI is 300ms or more, the subject sees a red flash followed by a green flash of equal durations.
He argued how these observations can be used to demystify a whole range of psychological phenomena. For example, each saccade (the blur between two fixations) is 40 ms. Above data may explain why we never see it (so called saccadic suppression).
He proposed that the principle of retina is "create the briefest possible optic nerve volley that includes all information in a scene, and send it off to cortex at the highest possible rate."
At the very end, he raised several intriguing questions: (a) If optic nerve volley is 300ms, how do we perceive motion when pictures are flashed at the rate of 24 Hz? (b) Observe that when ISI is less than 90 ms, the subject perceives orange. Why?