Neural Mechanisms That Hide Individual Behavioral Differences: Evidence from Psychophysics and Neuroscience

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In both theory and practice, individual behavioral differences can reveal details of underlying neural mechanisms, and this has been widely exploited in experimental psychology. However, under some circumstances, individual differences are conspicuous by their absence. Three illuminating examples are treated in this theoretical review: (1) In color vision, there is a surprising lack of variation in red–green color opponency, especially as studied using unique hues, given the huge variation of L:M-cone ratios in normal observers. (2) Conversely, in achromatic vision, individual differences in L:M-cone ratios can be studied by measuring spectral sensitivity (luminance efficiency) functions. However, contrary to reasonable expectations, parvo and magno mechanisms can give rise to indistinguishable spectral sensitivity functions, so individual variations in parvo and magno activation often cannot be studied via spectral sensitivity. (3) Similar convergences occur in neuroscience: in simulated and actual neuronal networks and in electrophysiological/functional imaging studies of intact animals/humans. Neuronal systems trained or developed to do the same tasks need not wind up with the same wiring or the exact same behavior. However, under some circumstances, their behaviors can become functionally similar. Markedly different neural mechanisms somehow yield similar behaviors, a result found in systems as different as motor behaviors in crustaceans and sensory behaviors in humans. Theoretically, similar neural mechanisms can result from different neural combinations, a response convergence which limits our ability to infer the origins of some perceptual channels. When expected individual differences do not manifest, this is a clue that something interesting is happening and a goad to further investigation.