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Lecture 30 1 The primary auditory cortex appears to be well organised with respect to frequency and carries on its surface a “map” of the cochlea, as is found in the subcortical nuclei. High frequency excitation, orignating in the base of the cochlea, is received in neurons located in the more medial portion of the primary area, deep in the lateral fissure. Low frequency information from the base of the cochlea is handled by neurons located more laterally. 2 Auditory information processing in the auditory cortex occurs due to a group of neurons organised in a vertical manner (columnar organisation). The most clear-cut parameter along which this organisation has been observed is the characteristic frequency of the nerve cells. Those neurons are sharply selective to one frequency of stimulation tend to the same characteristic frequency if they lie within the same column The nerve cells of the auditory cortex appear to reflect increased specialisation. The nerve cells become increasingly selective in their response to novel stimuli or certain features of the stimulus. 3 Which functions can still be accomplished (or relearned) when the auditory cortex has been removed? Which ones can be carried out only when the cortex is intact? These questions have been investigated in laboratory animals, particularly cats. The approach has been to bilaterally ablate the auditory cortex and then test the animals ability to perform or relearn various sound discrimination tasks. 4 There are some tasks that can be accomplished after bilateral ablation: 1. The onset of the sound 2. Changes in the tonal intensity 3. Changes in the frequency of the tonal stimulus *however, the type of the frequency discrimination is critical especially when the discrimination is between sequence of tones 5 Bilaterally ablated animals cannot discriminate: 1. Changes in the tonal duration 2. Changes in the temporal pattern of a tonal sequence 3. Sound localisation in space • Neff (1967) suggested that the auditory cortex plays an important role in the accurate localisation of sounds in space. 6 Most studies on humans with central auditory lesions have employed speech tests. Bilateral temporal lobe damage in humans have shown to result in impaired ability to make temporal pattern discriminations. Temporal lobe damage in humans has also been reported to result in impaired sound localisation in space. These results confirm the animals studies. 7