PATTERNS OF SYNAPTIC ACTIVITY IN FORWARD AND FEEDBACK PATHWAYS WITHINRAT VISUAL-CORTEX

1. The laminar and temporal distribution of synaptic activity supplied by forward and feedback connections between different areas of rat vi sual cortex was determined with the use of current source density (CSD ) analysis in in vitro slices. In forward connections, synaptic potent ials were evoked by electrically stimulating area 17 and recording in the extrastriate area LM (lateromedial), that ranks at the second hier archical level, one step above primary visual cortex. For activating f eedback connections, the location of stimulating and recording electro des was reversed. 2. The synaptic interactions in reciprocal intracort ical circuits are excitatory, and they are mediated through glutamate receptors that are blocked by kynurenic acid. 3. Forward connections f rom area 17 to area LM provide input to all layers including a strong input to layer 4. In contrast, feedback input to layer 4 is weak and i s mainly directed to superficial and deep layers. This laminar distrib ution closely resembles that seen anatomically. 4. Both forward and fe edback connections evoke distinct temporal patterns of synaptic activa tion in different layers. Although onset and peak latencies are slight ly shorter in the forward than in the feedback pathway, the difference is not statistically significant. 5. The spatiotemporal distribution of synaptic activation by forward connections resembles the pattern ev oked by geniculocortical inputs. Feedback connections show greater sim ilarities to long-range connections within area 17, although they are not identical. Our results support the notion derived from anatomic an d in vivo physiological studies that forward and feedback pathways bel ong to functionally distinct cortical circuits.