Introduction

In the previous chapter, we reasoned from models and confirmed with data that the tuning of V1 cells to Glass patterns could largely be explained by understanding a neuron's response to a translational Glass pattern confined to the CRF. This understanding extends from linear filter models of V1 receptive fields, which were generated and refined primarily based on data collected with oriented bars and sinusoidal gratings.

The salient percept of global form generated by Glass patterns has led to numerous psychophysical studies (Prazdny, 1984; Wilson and Wilkinson, 1998; Dakin, 1997a; Glass and Switkes, 1976; Ross et al., 2000; Prazdny, 1986; Earle, 1985; Dakin and Bex, 2001; Wilson et al., 1997; DeValois and Switkes, 1980). As we outlined in the previous chapter, Glass patterns would appear to be processed in two stages. The first stage would identify local orientation cues in the pattern, and the second stage would integrate these local signals to extract global form information. In Chapter 3 we have shown that V1 neurons are capable of acting as the first stage. However, based on the results of our experiments with Glass pattern size, V1 neurons do not appear to provide signals related to the global form of the pattern. Some recent psychophysics has provided evidence for a type of pooling that would be dependent on this second stage (Wilson and Wilkinson, 1998; Wilson et al., 1997) (although a recent study by Dakin and Bex (2002) has questioned these findings). Glass patterns have typically been studied using extended displays of 10$^\circ$ or more in diameter, positioned at the center of gaze. Our earlier results showed that the responses of neurons in V1 to these large patterns are predictable from those to a pattern confined to the CRF, which could see only a small portion of such a large Glass pattern. The next step in understanding the neural signals underlying the perception of Glass patterns would therefore require recordings in higher visual areas involved in form perception. Physiological data has shown that macaque V2 (Hegdé and Van Essen, 2000; Peterhans and von der Heydt, 1993) and V4 (Pasupathy and Connor, 2001; Gallant et al., 1996,1993; Pasupathy and Connor, 1999) contain neurons sensitive to complex form information present in a stimulus. We therefore decided to continue our investigation into Glass pattern perception in V2, an area which is known to respond well to conventional grating stimuli (Levitt et al., 1994) but differ from V1 in its responses to some more complicated stimuli (Anzai and Van Essen, 2001; Lee et al., 2002; Mahon and DeValois, 2001; Zhou et al., 2000).

Previous studies in macaque V2 with complex form stimuli have primarily confined them to the CRF. Because concentric and radial Glass patterns contain dot pairs which form curved edges and junctions similar to those found in previously used stimuli (Hegdé and Van Essen, 2000; Pasupathy and Connor, 2001; Peterhans and von der Heydt, 1993; Pasupathy and Connor, 1999; Gallant et al., 1993,1996), finding selective neuronal responses to these type of Glass patterns within the receptive field would merely replicate these results. However, psychophysical experiments with Glass patterns have used a field of dots which extends well beyond the CRF of an individual V2 cell. Furthermore, a small patch of any Glass pattern is approximated well by a translational pattern (Fig. 3-1). We wondered whether the form information present in the extended pattern would be able to modulate the response of V2 neurons when the pattern presented in the CRF was the same.

In this chapter, we report on the responses of individual neurons in V1 and V2 of macaque monkeys to Glass pattern stimuli. We first compare their responses to translational Glass patterns using similar analysis to that in Chapter 3. In some V2 cells, we then presented a stimulus designed to test their sensitivity to global form information presented outside the classical receptive field.