lunedì 22 luglio 2013

Cervello destro e sinistro. The useless stairway for decryption of smell.

La scala che conduce alla decrittazione di un odore ha molti scalini, ma l'olfatto pare salire con la corda direttamente dall'androne.

                                                                                   (Jicky 2.0).

La rapidità del coinvolgimento olfattivo non appare mediata.

Che ruolo ha in questa modalità percettiva il talamo (che in questo caso non funge da filtro)?

Cervello destro e sinistro

Due emisferi

La corteccia cerebrale è divisa in due emisferi dx e sx, collegati dal corpo calloso, che fornisce un "autostrada dell'informazione" tra loro.
Le due metà non sono immagini speculari né contengono funzioni del tutto esclusive. Ciò nonostante, ci sono somiglianze significative. Ogni metà riceve informazioni sensoriali però, stranamente, dal lato opposto del corpo. Così l'occhio destro va al cervello sinistro e viceversa. L'eccezione è il naso: la narice destra va al cervello destro.
Funzioni lateralizzati, invece, si trovano principalmente in un emisfero. L'emisfero dominante per la maggior parte della gente di mano destra questo è l'emisfero sinistro. Per molte persone mancine vi è una inversione, e l'emisfero dominante si trova sulla destra. Quando si parla di 'cervello sinistro', di solito intendiamo la emisfero dominante.
Respirazione attraverso la narice sinistra non interessa quindi l'attività cerebrale corticale sul lato destro, come invece avviene per l'occhio sinistro. Emisfero destro è associato a f(x) emozionali, visive, "femminili" (anche nel maschio).
Mentre l'emisfero sinistro è associato ad attività verbale e logica.
Il naso parrebbe l'eccezione, unico senso che non utilizza il rapporto chiasmico.

Olfactory perceptual learning: the critical role of memory in odor discrimination

Donald A Wilsona, Richard J Stevensonb,
a Department of Zoology, University of Oklahoma, Norman, OK 73019, USA
b Department of Psychology, Macquarie University, Sydney, NSW, Australia
http://dx.doi.org/10.1016/S0149-7634(03)00050-2, How to Cite or Link Using DOI

The major problem in olfactory neuroscience is to determine how the brain discriminates one odorant from another. The traditional approach involves identifying how particular features of a chemical stimulus are represented in the olfactory system. However, this perspective is at odds with a growing body of evidence, from both neurobiology and psychology, which places primary emphasis on synthetic processing and experiential factors—perceptual learning—rather than on the structural features of the stimulus as critical for odor discrimination. In the present review of both psychological and sensory physiological data, we argue that the initial odorant feature extraction/analytical processing is not behaviorally/consciously accessible, but rather is a first necessary stage for subsequent cortical synthetic processing which in turn drives olfactory behavior. Cortical synthetic coding reflects an experience-dependent process that allows synthesis of novel co-occurring features, similar to processes used for visual object coding. Thus, we propose that experience and cortical plasticity are not only important for traditional associative olfactory memory (e.g. fear conditioning, maze learning, and delayed-match-to-sample paradigms), but also play a critical, defining role in odor discrimination.

Olfactory memory; Perceptual learning; Piriform cortex; Olfactory bulb; Olfactory coding; Odor discrimination; Olfactory psychophysics; Synthetic coding
Figures and tables from this article:

Odorants are hypothesized to be composed of multiple features (B, C, D, E, F) that are each recognized by phenotypically different olfactory receptor neurons (ORN). Contrast between features is enhanced through convergence and lateral interactions in the main olfactory bulb (MOB). Features are bound into odor objects through temporal synchrony and anatomic convergence within the anterior piriform cortex (aPCX) and potentially other cortical structures. (Bottom) Representation of the primary neuroanatomical structures implicated in rodent and human olfaction.
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Fig. (...) Data adapted from Livermore and Laing, [83] on the ability of olfactory experts and recently trained controls to identify components within an odor mixture (odor analysis). The data demonstrate two important points: (1) experts and non-experts have the same identification ceiling, i.e. analysis breaks down as mixture complexity increases beyond 3–4 components; (2) experts perform better than non-experts below the ceiling, i.e. evidence of a form of olfactory perceptual learning.
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Fig. (...) On any given inhalation, multiple odorants composed of multiple features will activate olfactory receptors and their central targets. In order to discriminate between odors or detect an odor against a background, perceptual organization (synthesis) of features belonging to the target odor (‘dog’) must occur, similar to what is needed for the visual organization task shown here. In both vision and olfaction, prior experience with the target stimulus enhances recognition of the target stimulus (perceptual learning). This grouping of ‘dog’ odor components, however, will greatly impair identification of individual components within the ‘dog’ odor mixture. Original photo by R.C. James.
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Fig. (...) The ability of aPCX neurons to discriminate binary mixtures of dissimilar odorants from their components is dependent on the duration of exposure to the mixture. The bottom panel displays an intracellularly recorded aPCX neuron response to a 50 s odorant presentation. The response rapidly changes over the first 10–15 s, and is then largely suppressed for the remainder of the stimulus. Tests of cross-habituation between the binary mixture and its components (top panel) after 10 s of exposure to the mixture reveals similar amounts of self- and cross-habituation, i.e. failure to discriminate. Tests of cross-habituation after 50 s of exposure to the mixture (in different cells and animals) reveals significantly less cross-habituation, i.e. the cell has learned to discriminate between the mixture and its components. Mitral/tufted (M/T) cells however, fail to discriminate between the mixture and its components, consistent with the feature detecting role hypothesized for these odors. From Wilson, [176].

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