MIT Encyclopedia of the Cognitive Sciences - Cryptome

Neurosciences lxi
the correlation between a feature of the neuronal response (typically magnitude) and
some physical parameter of a sensory stimulus (such as the wavelength of light or the
orientation of a contour). Because the perceptual interpretation of a sensory event is
necessarily context-dependent, this approach alone is capable of revealing little, if
anything, about the relationship between neuronal events and perceptual state. There
are, however, some basic variations on this approach that have led to increased understanding
of the neuronal bases of perception.
Experimental Approaches to the Neuronal Bases of Perception
Origins of a Neuron Doctrine for Perceptual Psychology The first strategy involves
evaluation of neuronal responses to visual stimuli that consist of complex objects of
behavioral significance. The logic behind this approach is that if neurons are found to
be selective for such stimuli, they may be best viewed as representing something of
perceptual meaning rather than merely coincidentally selective for the collection of
sensory features. The early studies of “bug detectors” in the frog visual system by
Lettvin and colleagues (Lettvin, Maturana, MCCULLOCH, and PITTS 1959) exemplify
this approach and have led to fully articulated views on the subject, including the
concept of the “gnostic unit” advanced by Jerzy Konorski (1967) and the “cardinal
cell” hypothesis from Barlow's (1972) classic “Neuron Doctrine for Perceptual Psychology.”
Additional evidence in support of this concept came from the work of
Charles Gross in the 1960s and 1970s, in the extraordinary form of cortical cells
selective for faces and hands (Gross, Bender, and Rocha-Miranda 1969; Desimone et
al. 1984). Although the suggestion that perceptual experience may be rooted in the
activity of single neurons or small neuronal ensembles has been decried, in part, on
the grounds that the number of possible percepts greatly exceeds the number of available
neurons, and is often ridiculed as the “grandmother-cell” hypothesis, the evidence
supporting neuronal representations for visual patterns of paramount
behavioral significance, such as faces, is now considerable (Desimone 1991; Rolls
1992).
Although a step in the right direction, the problem with this general approach is
that it relies heavily upon assumptions about how the represented information is used.
If a cell is activated by a face, and only a face, then it seems likely that the cell contributes
directly to the perceptually meaningful experience of face recognition rather than
simply representing a collection of sensory features (Desimone et al. 1984). To some,
that distinction is unsatisfactorily vague, and it is, in any case, impossible to prove that
a cell only responds to a face. An alternative approach that has proved quite successful
in recent years is one in which an effort is made to directly relate neuronal and perceptual
events.
Neuronal Discriminability Predicts Perceptual Discriminability In the last quarter
of the twentieth century, the marriage of single-neuron recording with visual psychophysics
has yielded one of the dominant experimental paradigms of cognitive neuroscience,
through which it has become possible to explain behavioral performance on
a perceptual task in terms of the discriminative capacity of sensory neurons. The earliest
effort of this type was a study of tactile discrimination conducted by Vernon
Mountcastle in the 1960s (Mountcastle et al. 1967). In this study, thresholds for
behavioral discrimination performance were directly compared to neuronal thresholds
for the same stimulus set. A later study by Tolhurst, Movshon, and Dean (1983)
introduced techniques from SIGNAL DETECTION THEORY that allowed more rigorous
quantification of the discriminative capacity of neurons and thus facilitated neuronalperceptual
comparisons. Several other studies over the past ten years have significantly
advanced this cause (e.g., Dobkins and Albright 1995), but the most direct
approach has been that adopted by William Newsome and colleagues (e.g., Newsome,
Britten, and Movshon 1989). In this paradigm, behavioral and neuronal events
are measured simultaneously in response to a sensory stimulus, yielding by brute
force some of the strongest evidence to date for neural substrates of perceptual discriminability.