SALAMANDER OLFACTORY-BULB NEURONAL-ACTIVITY OBSERVED BY VIDEO-RATE, VOLTAGE-SENSITIVE DYE IMAGING .2. SPATIAL AND TEMPORAL PROPERTIES OF RESPONSES EVOKED BY ELECTRIC-STIMULATION

1. Video imaging of changes in voltage-sensitive dye (VSD) fluorescenc e was used to analyze spatial and temporal properties of activity patt erns in the in vivo salamander olfactory bulb and primordium piriform cortex after electric stimulation. Distribution of activity among and within the neuronal layers was analyzed after orthodromic stimulation of the whole olfactory nerve (ON), isolated fascicles, or local epithe lial sites, and after antidromic stimulation of the medial olfactory t ract (OT). 2. Optical signals propagated through the bulbar layers wit h a sequence that correlates with electrophysiological responses. Afte r orthodromic stimulation, VSD responses started in the glomerular lay er, spread to the deeper laminae, and, after reaching the region of mi tral/tufted somata, were observed as a brief burst of activity in the OT. Compound action potentials in the ON were associated with short-du ration, rapidly depolarizing optical responses in the ON layer. Respon ses in glomerular layer and external plexiform layer (EPL) first showe d in some recordings a brief, small-amplitude hyperpolarization, follo wed by a period of depolarization, followed by a second, longer-lastin g hyperpolarization. The periods of optical hyperpolarization could be related to events observed in intracellular mitral/tufted cell record ings. 3. With shocks delivered to the entire ON, depolarizing response s were nonhomogeneously distributed, appearing as multiple foci or ban ds of activity. Spatial patterns within each bulbar layer had poorly d efined borders. Sites showing short-latency responses were often those with the largest and longest-lasting activity. 4. Increasing the inte nsity of stimulation to the ON enhanced the size and duration of the d epolarizing and hyperpolarizing responses. The short-latency, early hy perpolarization was best seen with low-intensity, peripherally placed stimuli. 5. ON stimulation also elicited activity in the contralateral bulb. Activity started at the innermost layers and spread in patches to regions of the EPL just beneath the glomeruli. These had durations similar to ipsilateral responses, but longer latencies. A period of ea rly hyperpolarization, longer than that on the ipsilateral side, was f ollowed by prolonged depolarization and then by a second, later hyperp olarization. 6. Antidromic stimuli applied to the OT evoked optical re sponses consisting of a period of depolarization followed by hyper pol arization, similar to the components elicited by orthodromic stimuli. These responses had shea time courses, began in the deeper layers, and spread to the superficial region of the bulb usually without reaching the glomerular region. 7. Punctate stimulation of the mucosa or nerve elicited depolarizing and hyperpolarizing events that depended on the stimulation site. However, the spatial distribution of activity was c omplex and not based on a simple topographic mapping of the mucosa ont o the bulb. Local stimulation of restricted epithelial sites activated relatively large bulbar regions and patterns often showed overlap wit h one another, although there was a relatively consistent relationship between the mucosa and bulb across animals. 8. Examination of the rel ationships among these spatially distributed patterns provides support for the hypothesis that the connections between the mucosa and bulb c onsist of complex convergent and divergent projections and that this c omplexity may itself be important for encoding and integrating odorant information. These data form the basis for examining responses to odo rant stimulation presented in the following paper.