1. This is the second of a series of papers on the electrosensory lobe and closely associated strucutres in electric fish of the family Mormyridae. The focus of the study is on the regions of the electrosensory lobe where primary afferent fibers from mormyromast electroreceptors terminate. 2. This second paper examines the responses of single cells in the mormyromast regions of th electrosensory lobe to electrosensory stimuli and to corollary discharge signals associated with the motor command that drives the electric organ to discharge. All recordings were extracellular. 3. Two major types of cells were identified: I cells, which were inhibited by electrosensory stimuli in thecenter of their receptive fields; and E cells, which were excited by such stimuli. 4. I cells and E cells shared a number of common features. Both types could have small receptive fields limited to only a few electroreceptors (3-5), and both types were markedly affected by the corollary discharge of the elctric organ discharge (EOD) motor command. Cells of both types also showed clear plasticity in their responses to the corollary discharge or to the corollary discharge plus a stimulus. 5. I cells could be subdivided into three subtypes, I1, I2, and I3, on the basis of corollary discharge repsonses in the absence of sensory stimuli. I1 and I2 cells showed consistent corollary discharge bursts with little or no additional activity beyond the duration of the burst. The corollary discharge bursts of I1 cells were more stereotyped and of shorter latency than those of I2 cells. I3 cells had more spontaneous activity than I1 or I2 cells and minimal corollary discharge responses in the absence of sensory stimuli. Field potentials indecated that all three subytpes of I cells were recorded in or near the ganglion layer of the electrosensory lobe. 6. Corollary discharge responses were plastic and depended on recent pairing of a sensory stimulus with the EOD motor command. Such plasticity was clearer in I2 and I3 cells than in I1 cells. Inhibitory sensory stimuli were paired with the EOD motor command for periods of a few seconds to several minutes. Such pairing resulted in a markded enhancement of the corollary discharge response in I2 cells, as shown by examining the effect of the motor command after turning off the stimulus. In I3 cells, such pairing resulted in a clear corollary burst to the command at the time of the previously paired inhibition. The altered corollary discharge responses of both cell types slowly returned to the prepaiting level over a period of 2-3 min after turning off the paired stimulus. 7. E cells also appearted to be heterogeneous group but could not be divided into distinct subtypes in these experiments. Most E cells showed little spontaneous activity or corollary dischatge effects in theabsence of sensory stimulation, but responses to sensory stimuli were markedly facilitated by the corollary discharge. Sensory stimuli that elicited little or no response at long delays with respect to the command elicited vigorous responses when given at short delys. Field potentials indicated that E cells were recorded in several different layers of thelectrosensory lboe. 8. Plasticity of corollary discharge faclitation in E cells was shown by pairing the EOD motor command with a sensory stimulus at a short delay. The response to the cammand plus the paired stimulus showed a marked decline during pairing periods of 2-3 min. Such declines did not occur during control stimulus periods in which stimui were given at long edlays or independently of the command. Thus the marked declines furing pairings at short delays were not due to a simple decrease in sensory responsiveness. 9. Two different time courses or phases of plastic change could be distinguised in both I and E cells: a rapid phase in which clear changes took place within a few seconds after stimulus-on or stimulus-off, and a slower pase taking several minutes to reach a maximum after stimulus-on, or to disappear after stimulus-off. 10. Previous behavioral studies have indicated two different functional roles for corollary discharge signals in active electrlocation: 1) selective enchancement of the (re)afferent sensory input that is evoked by the fish's own EOD and that informs the fish about nearby conductance; and 2) provision of a temporal reference point for measuring the latencies of EOD-evoked afferent input allowing such latencies to serve as a code for stimulus intensity. The physiological findings reported here provide strong support for both of these functional roles. 11. Corollary discharge plasticity is a prominent feature of the mormyromast regions of the mormyrid electrosensory lobe, but the functional role for such plasticity has not been determined. However, various behavioral and physiological observations suggest two possible roles for corollary discharge palsticity: 1) adjustment of the operating point of the system to changes in EOD amplitude and 2) detection of novel stimuli.
