1 Reverse Correlation and the VESPA Method

The traditional method of obtaining event-related potentials (ERPs) typically involves the repeated presentation of discrete stimulus events and extraction from the ongoing neural activity using signal averaging techniques. Assuming a sufficiently high sam pling rate, this technique allows for the determination of a response whose individual components are clearly resolved in time, allowing for a temporally detailed analysis of sensory and cognitive processes. While this method has led to tremendous advances in our understanding of the brain, both healthy and otherwise, it has a number of intrinsic limitations. These include the inability to adequately resolve responses to more than one stimulus at a time, the nonenvironmental and somewhat aversive nature of suddenly onsetting stimuli, particularly in the visual domain, and the lengthy acquisition time resulting from the incorporation of a sufficient delay between stimuli to allow the neural activity to return to baseline. In this chapter we describe a method for obtaining a novel visual ERP known as the VESPA (for visual evoked spread spectrum analysis) that seeks to address the limitations of the standard visual evoked potential (VEP). This method involves the recording of neural activity during the presentation of continuous, stochastically modulated stimuli and the use of reverse correlation to determine the transfer function of the human visual system, that is, the function that converts the presented stimulus into the recorded neural activity. First, we introduce the broader reverse correlation technique that has seen widespread use in the analysis of many physiological systems. We follow this introduction with a description of the VESPA method itself, including a discussion of the differences between the standard VEP and the VESPA, and some proposed applications and extensions.