One obstacle to the electrophysiological study of form vision has been the difficulty in choosing a stimulus which proves effective in eliciting responses from higher cortical areas and can be varied quantitatively. The combination of these two properties appears necessary to determining the basis of receptive field structure in these cortical areas. Some attempts at this approach have involved the use of concentric or radial grating stimuli (Hegdé and Van Essen, 2000; Gallant et al., 1993,1996), while others have used a set of object ``primitives'', consisting of corners and junctions (Kobatake and Tanaka, 1994; Pasupathy and Connor, 1999). V2 has received attention as an area that might play an important role in form vision with its strong inputs from V1 and outputs to V4 and other ventral visual areas. Our previous results (see Smith et al. (2002) and Chapter 3) show that V1 is capable of providing the fundamental signals necessary for the detection of structure in Glass patterns. We sought to record in V2 in order to determine how these signals are combined and further processing takes place.
We might expect V2 neurons to be more responsive or sharply tuned to translational Glass patterns because of the potential for combining inputs from V1 cells. However, the distribution of various response properties to gratings is similar in V1 and V2 (Levitt et al., 1994), which might lead us to expect similar Glass pattern responses in V1 and V2. Our data support the latter hypothesis. When using translational Glass pattern stimuli, we found a striking similarity in response properties in V1 and V2. The orientation tuning preference, optimal dot separation and modulation in firing rate were largely indistinguishable between V1 and V2.
Our stimulus design for these experiments proved to be useful for collecting large amounts of data in a relatively small amount of time. When comparing results in V1 between the old and new methods, we found very little difference. This leads us to conclude that under some circumstances, a continuous stimulus presentation like the one we used may be an effective method for collecting reliable data, particularly in situations where the responses are noisy and a large number of repeats is desirable.
One major question about the neural processing of form perception is whether it is accomplished through neurons with a large CRF sensitive to the form within it or by mechanisms outside the CRF. While neurons with sensitivity to complex shapes within their CRF have been reported in multiple studies, it is also the case that surround effects have been found by many researchers in multiple visual areas. It is with this latter possibility in mind that we performed our experiments using the full-screen concentric, radial and translational Glass patterns. Furthermore, different Glass pattern stimuli have been shown to elicit percepts of global form with different strength (Wilson and Wilkinson, 1998; Wilson et al., 1997). This leads us to believe that individual neuronal responses to Glass patterns in some visual area might reflect these psychophysical results.
Despite these expectations, we found that responses to concentric and radial Glass patterns in our stimuli were similar to those to a preferred translational pattern. This is what we would expect if V2 neurons merely responded to the dots that fell within the CRF, because the concentric and radial Glass patterns were positioned to imitate a translational Glass pattern within the CRF of the cell. With this paradigm, we were unable to find evidence that V2 neurons could distinguish between different global forms in their surround. However, they did show surround suppression to large gratings, indicating that some signals from these regions of visual space are influencing the neuron's response.
There are four possible reasons for this apparent lack of sensitivity to global form in V2 cells. First, it may be that anesthesia affects the sensitivity of V2 neurons to global form stimuli. There is some precedent for this - Lamme et al. (1998) found differences in figure-ground segregation between awake and anesthetized animals. Second, Glass pattern stimuli may not sufficiently drive circuits that are present in V2 and able to detect global form outside the CRF. Third, it is possible that neurons in V2 may not be able to detect global form outside the CRF, but neurons in some higher visual area may be able to do so. Fourth, perhaps the computation of global form actually does take place within the large CRFs of cells in higher levels of the visual system. Neurons in V4 and IT have shown sensitivity to complex form stimuli presented within their CRF, and have significantly larger receptive fields than those found in V1 or V2. A study of their responses to Glass pattern stimuli presented inside and outside their CRF might help reveal the mechanisms of Glass pattern perception and determine the relative contributions of processing inside and outside the CRF.