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) fluorescence was used to analyze spatial and temporal properties of activity patterns 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 epithelial sites, and after antidromic stimulation of the medial olfactory tract (OT). 2. Optical signals propagated through the bulbar layers with a sequence that correlates with electrophysiological responses. After orthodromic stimulation, VSD responses started in the glomerular layer, spread to the deeper laminae, and, after reaching the region of mitral/tufted somata, were observed as a brief burst of activity in the OT. Compound action potentials in the ON were associated with short-duration, rapidly depolarizing optical responses in the ON layer. Responses in glomerular layer and external plexiform layer (EPL) first showed in some recordings a brief, small-amplitude hyperpolarization, followed by a period of depolarization, followed by a second, longer-lasting hyperpolarization. The periods of optical hyperpolarization could be related to events observed in intracellular mitral/tufted cell recordings. 3. With shocks delivered to the entire ON, depolarizing responses were nonhomogeneously distributed, appearing as multiple foci or bands of activity. Spatial patterns within each bulbar layer had poorly defined borders. Sites showing short-latency responses were often those with the largest and longest-lasting activity. 4. Increasing the intensity of stimulation to the ON enhanced the size and duration of the depolarizing and hyperpolarizing responses. The short-latency, early hyperpolarization 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 early hyperpolarization, longer than that on the ipsilateral side, was followed by prolonged depolarization and then by a second, later hyperpolarization. 6. Antidromic stimuli applied to the OT evoked optical responses consisting of a period of depolarization followed by hyper polarization, 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 complex and not based on a simple topographic mapping of the mucosa onto the bulb. Local stimulation of restricted epithelial sites activated relatively large bulbar regions and patterns often showed overlap with one another, although there was a relatively consistent relationship between the mucosa and bulb across animals. 8. Examination of the relationships among these spatially distributed patterns provides support for the hypothesis that the connections between the mucosa and bulb consist of complex convergent and divergent projections and that this complexity may itself be important for encoding and integrating odorant information. These data form the basis for examining responses to odorant stimulation presented in the following paper.