Neuroscience for Psychologists by Unknown

Author:Unknown
Language: eng
Format: epub
ISBN: 9783030476458
Publisher: Springer International Publishing

6.3 “Hierarchy” in the NS

In publications that intend to model brain functioning, we frequently read of bottom-up or top-down signaling referring to a type of hierarchy. Now, what is the meaning here of “high” or “low” levels? In neuroscience, we consider parts of the CNS as anatomically low and high in the hierarchy along a vertical dimension if a human is in an upright position. This means that the medulla of the spinal cord is lower than the medulla oblongata, lower than the metencephalon, mesencephalon, diencephalon, basal ganglia and cortices. However, in the neocortex, this hierarchy does not apply: The temporal cortex is not “lower” than the parietal cortex, even though in the upright human, it is definitely below the parietal cortex. Another hierarchy is defined in sensory pathways, such as the visual and auditory. In vision, the retina is “below” the thalamus which is “below” the visual cortex, because, by and large, signal transmission is from retina to thalamus and from there to the cortex. Such pathways can be defined for the other exteroceptive senses, too.

From the very beginning, i.e., the level of receptors, processing of stimulus-generated signals is influenced from “above”, i.e., “inputs” are not really a “one-way street” but depend on the history and of states of the nervous system “above.” An example is shown in ◘ Fig. 6.1, the sound attenuation reflex. In this case, the top-down adjustment goes all the way to the structures involved in capturing the stimulus. In particular, in the visual and auditory pathways, “top-down” regulation has been found at any level. Part of that regulation from “above” serves to filter “hoped-for” or expected features, a process necessary to produce “attention.” One example is spatial attention: When we expect a certain event in the right visual field, visual neurons encoding the right visual field will fire at a higher baseline level. Higher baseline activity due to attention can be found in neurons low in the visual hierarchy, such as in the thalamus, and in neurons in higher regions, such as in the primary visual cortex (O'Connor et al. 2002).

Fig. 6.1The sound attenuation reflex. An example of modification of the incoming stimulus “from above”: Auditory information is transmitted from the organ of Corti via the central processes of bipolar sensory neurons to the ipsilateral ventral cochlear nucleus. Subsequent connections are bilateral. Information is then transmitted sequentially to the superior olivary nucleus which in turn projects to the trigeminal motor and facial motor nuclei. The trigeminal motor nucleus projects to the tensor tympani and the facial motor nucleus projects to the stapedius muscle, causing their contraction, thus dampening the auditory input. FN facial nucleus, TMN trigeminal motor nucleus, SON superior olivary nucleus, VCN ventral cochlear nucleus, SG spiral ganglion, OC organ of Corti, OW oval window, T tympanum, S stapes

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