Neuro103_chp8.doc

BN103 Study Questions Chapter 8: Acoustic Object Analysis What properties of the basilar membrane allow it to function as a sound-frequency analyzer? the basilar membrane in a fluid environment create a tonotopic map of frequency along the basilar membrane different areas of the basilar membrane are displaced by specific frequencies; the base is displaced by higher frequencies, the apex is displaced by lower frequencies Distinguish between the ?place? and ?volley? theories of frequency coding by the cochlea. Place coding: frequencies are absorbed by distinct areas of the basilar membrane Volley coding: action potentials in nerve axons that are phase-locked to the signal occur at identical times in each cycle, producing volleys of impulses that collectively have the same period as the acoustic signal E.g. a sound wave activates 3 nerves but the APs are phase locked to the sound. Together, the discharge of three nerves faithfully reproduces the frequency of the sound wave What is meant by the ?characteristic frequency? of a cell in the auditory pathway? best frequency of the cell the frequency that has the lowest threshold, therefore, highest response from the cell How might you define the ?receptive field? of an auditory system neuron on the basis of its response to tones of various frequencies? Tuning curves of the cells in the space of acoustic frequency If one records from several axons in the nerve and plots their spectral thresholds, varying the frequency of the stimulus and plotting minimal intensity of sound required to produce a response produces different tuning curves As early as the VIIIth nerve, frequency information is distributed among a number of fibers (as a population or ensemble code) What is the evidence for distributed or ensemble coding of acoustic frequency in the primary auditory afferents? Frequency is signaled by the location and composition of the patch of excited fibers in the nerve As stimulus increases in intensity, more axons are recruited When frequency shifts, the patch changes position in the population of auditory nerve fibres Neighboring tones are encoded by overlapping populations of nerves Compare and contrast the spatial organization of the primary auditory cortex and the primary visual cortex. Visual system: the code in the photoreceptors is retinotopic; the activity of a given photoreceptor indicates light at a particular location Auditory system: receptors establish a spatial code that is tonotopic because of the unique properties of the cochlea Retinotopy or tonotopy is preserved as signals are processed in secondary neurons, in the thalamus, and finally in sensory cortex What are the major cues available to the auditory system about the direction of a sound source? the ?shadow? produced by the head results in intensity differences at the two ears Source off the midline reaches one ear slightly before the other Time delays in the microsecond range are detectable by the nervous system Pinna filters information about elevation of sound, but good localization in elevation requires the head moving to zero in on source What areas of the brain are thought to be important in sound localization and why? Auditory cortex sensitive to minute delays and intensity differences Superior olive and inferior colliculus important in the reflex orientation to stimuli How would you define acoustic object? Spectrotemporal pattern? individual words or syllables formed of distinct frequency, intensity and time patterns of frequency of signal over time What are the major features of Broca?s aphasia? Wernicke?s aphasia? Broca?s: Non-fluent aphasia Slow and labored speech; grammatical word and endings absent; high content words strung together; telegraphic quality Musical tasks carried out well Comprehension relatively intact Acutely aware of deficit and distressed Reading intact but laborious; writing abnormal like speech Lesion near posterior end of inferior frontal gyrus Wernicke?s: Fluent aphasia Excessive speech (logorrhea) with normal melody, empty content (thing, this, etc), jargon and jibberish Comprehension impaired; difficulty interpreting spoken and written words Writing disturbed like deficits in spoken language Patients euphoric and not disturbed; some paranoia Associated with lesion of the posterior superior temporal gyrus What are alternative names for these two types of aphasia? Wernicke?s: Fluent aphasia Broca?s: Non-fluent aphasia What are Broca?s area and Wernicke?s area? Broca?s area: posterior superior temporal gyrus Wernicke?s: posterior end of inferior frontal gyrus What is the evidence that language function is largely mediated by the left hemisphere? Penfield found that areas where electrical stimulation interfered with speech were only found in left hemisphere One corresponds to Broca?s area, one in posterior parietal lobe near Sylvian fissure, temporal lobe (last two in vicinity of Wernicke?s area) Left planum temporale is much larger because it is dominant for speech Give an example of how an aphasic might make a phonetic error. A phonemic error. A lexical error. phonetic error: native speaker of Italian substitutes a French or German ?r? for his normal roller ?r? phonemic error: keams instead of teams; gedree instead of degree Lexical error: anomia- difficulty finding words; word substitution -mother for father; overuse of non-specific, empty words- thing, be, like Syntactic error: infrequent use of function words (and, the, a, very, have) giving telegraphic quality Place coding: dominant mechanism for frequencies above 6000 Hz; Volley coding: for frequencies below 400 Hz Categorical perception: the decoding of acoustic objects recognizes and categorizes the patterns despite a wide variation in the physical signals themselves Characteristic frequency: best frequency at which a cell responds Best frequency: see characteristic frequency Aphasia: disturbances in language function Fluent aphasia: language disturbance where speech is fluent and comprehension is poor Non-fluent aphasia: language disturbance with difficulty speaking or repeating words but can understand language; motor aphasia Anomia: difficulty choosing words or finding words Anterior aphasia: see: non-fluent aphasia Posterior aphasia: see: fluent aphasia Planum temporale: area of superior surface of temporal lobe that is larger in the left than the right; critical for language Broca?s area: region of frontal lobe associated with Broca?s aphasia (see: non-fluent aphasia) Wernicke?s area: area on the superior surface of the temporal lobe between auditory cortex and the angular gyrus, associated with Wernicke?s aphasia (see: Fluent aphasia) Wada test: one cerebral hemisphere is anesthetizes to enable testing of the function of the other hemisphere Syntax: order of verbs, nouns, function words Lexicon: pool of learned words; from where to select the appropriate word forms for elements of idea Phoneme: constituent sounds; phonemic problems are capable of generating isolated sounds but make errors in assembling the sounds to form meaningful words (geen instead of green) Acoustic stream: a stream of words or syllables formed of: frequency, intensity and time Acoustic object: words or syllables Neurolinguist: goal to identify isomorphisms between models developed from the study of natural languages and neural mechanisms of the brain; attempt to learn about language from those experiments of nature that disturb language function Aphasiologist: study aphasias (disturbances in language function)