Figure 7.
Conceptual model of a level-invariant representation in A1. A, Diagram of a cortical sheet of neurons responding to a pure tone. Gray circles represent individual neurons, ordered by best frequency and best level, and grayscale fill represents response rate. When a pure tone (at 5 kHz, for example) is presented at a low level (0 dB; left), only O units tuned to low levels and low-threshold V units respond. In both cases, because of narrow frequency tuning, spread of activity is restricted to a small number of neurons. When sound level is increased (80 dB; right), O units tuned to this level start responding but the units tuned to low levels stop firing. The pattern of activity generated is just as restricted as low sound levels. However, activity spreads over a range of V units coding different frequencies and sound levels, leading to a loss of spectral resolution. B, If a linear frequency modulated sweep (gray bar indicates spectral extent of sweep; small black arrows indicate instantaneous frequency of sweep) is presented as the stimulus at low levels (0 dB; left), a tight packet of activity propagates with the sweep across the cortical surface (snapshots of the population at 50 and 100 ms into the sweep are shown). When level is increased (80 dB; right), activity packets are just as well resolved in the O unit population. In the V unit population, there is temporal and spectral degradation. The neuron highlighted in black, for example, is active over a duration of 50 ms, starting to fire before the sweep “reaches” its BF (computed at threshold) and firing well after the sweep has crossed its BF. C, Temporal precision of response is also affected by bandwidth. When an lFM sweep (black line) crosses the excitatory receptive field (shaded gray) of a narrowly tuned unit (bottom), response duration (double arrow) is short. However, for broadly tuned units (top), response is smeared out over time.