**4. Discussion**

#### **4.1. Gender-related asymmetry during color processing**

Overall, the gender-related differences could be summarized as follows: (1) in men, wave‐ length-differencing activity was enhanced at high luminance effect, conversely, in women, frequency-differencing activity was enhanced at low luminance effect. (2) In men, luminance effect varied exponentially with wavelength-differencing activity, while in women, luminance effect varied logarithmically with frequency-differencing activity. (3) The physiologic range of MFV in the MCA territories for the relationship between luminance effect and wavelengthdifferencing activity in men, was narrower than that for frequency-differencing activity in women. The gender-related differences in color processing, which comprised right hemi‐ sphere wavelength-differencing in men, but left hemisphere frequency-differencing in women, suggest that, color processing followed the same rule of lateralization as other major cognitive functions, such as facial processing [26, 27] and general intelligence [30], which implicated the right hemisphere cognitive style in men, and left hemisphere cognitive style in women. Prior studies have suggested that, the right hemisphere was implicated in color processing [20, 21, 22] and the left hemisphere in color memory [61, 62]. Functional magnetic resonance studies demonstrated that memory for colors activated the same cortical regions associated with color perception in the left fusiform gyrus [63, 64]. This has led some investi‐ gators to suggest that, color memory is a constructive process reflecting the synthesis of features each of which are processed in the different cortical regions [65-67]. The functional integration of color processing and luminance effect could provide a model for the study of the functional integration of color processing and color memory within the color space.

activation peak [52, 53]. Conversely, in the left hemisphere in women, at **C**-peaks, frequencydifferencing separated the peak for high frequency color (Blue) from low frequency color (Yellow), but was parallel to the luminance axis. Hence, the differentiation was processmapped only to frequency. Similar process-map model has been proposed for facial processing [26, 34]. It could be suggested that in the left hemisphere in women, there is a category-specific process-mapping system for retrieving color from memory. The latter would be consonant to

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The present work may offer some indication on the neurophysiologic mechanisms underlying vasomotor changes during color processing, which have remained largely unknown until now. The cerebral arteries are innervated by postganglionic nitrergic nerves, originating from the ipsilateral pterygopalatine ganglion, that tonically dilate cerebral arteries in the resting condition [70]. The observed changes in MFV in response to visual stimulations have been related to imbalance between sympathetic vasoconstrictor traffic and vasodilator effects of nitric oxide (NO) [11, 22, 71]. The NO released postsynaptically, diffuses back across the synaptic cleft, to act on the presynaptic terminals, causing increases in presynaptic glutamate release [70, 72], which could account for ipsilateral LTP [11, 72]. Others contend that LTP induced in the visual cortex in animal models is NO dependent [72]. The LTP and LTD recorded non-invasively using *f*TCDS, have shown neuroplasticity during color processing over several hours [11], and hence could be applied to the study of stroke rehabilitation and

Presuming that these mechanisms are present, it could be proposed that in men, wavelengthdifferencing within the right hemisphere by processes of cortical long-term depression (CLTD) and subcortical long-term potentiation (SLTP) [12, 17], occurred simultaneously with con‐ tralateral cortical short-term depression (CSTD) and subcortical short-term potentiation (SSTP) in the left hemisphere, marked by exponential decaying increase in synaptic strength that appears to involve NMDA receptors as well [12, 17, 73, 74], but decays after reaching the asymptotic levels of MFV (Figure 5A-B). Thus, it could be postulated that in men, in the contralateral left hemisphere, memory activation implicated 'exponential expansion' by CSTD and SSTP processes. On the other hand, in women, frequency-differencing within the left hemisphere occurred by processes of cortical long-term potentiation (CLTP) and subcortical long-term depression (SLTD) [11, 12, 17], concurrently, with ipsilateral cortical short-term potentiation (CSTP) and subcortical short-term depression (SSTD) in a selective area of the left hemisphere [11, 12, 17], characterized by logarithmic decaying decrease in synaptic strength that may also involve NMDA receptors, but decays after reaching the asymptotic levels of MFV (Figure 5C). Thus, in women, in the ipsilateral left hemisphere memory activation involved 'logarithmic compression' by CSTP and SSTD processes. Analogous synaptic and

The clinical relevance of the observed profound gender-related differences in cortico-subcort‐ ical activation during color stimulation is yet to be fully realized. The plausible implications of the result that, in men, in the right hemisphere, SLTP and CLTD occurred with wavelengthdifferencing, and conversely, in women in the left hemisphere, CLTP and SLTD occurred with frequency-differencing, is not known. It could be suggested that, there could be a gender-

the proposed distinct map for representation of color in memory [66, 67].

monitoring of drug effects on N-Methyl- D-aspartate (NMDA) receptors.

cellular activities have been observed in animal experiments [73, 74].

However, the reason for gender-related cerebral asymmetry for color processing remains to be elucidated. It was suggested that, the neuroadaptation to the physical qualities of light as a wave and as quanta energy, pre-conditioned the right hemisphere for wavelength-differ‐ encing and left hemisphere for frequency-differencing, respectively. A light hypothesis for cerebral asymmetry was postulated, to infer that, the phenotypic neuroadaptation to the environmental physical constraints of light, led to phenotypic evolution and genetic variation of X-Y gene pairs that determined hemispheric asymmetry [12, 17]. The evolutionary trend is towards optimization of perception of the 'whole' environment by functional coupling of the genes for complementarity of both hemispheres within self, and between both genders [12, 17, 26]. The contralateral hemisphere processing of wavelength-differencing and frequencydifferencing was as a result of the inverse relationship between wavelength and frequency. The results further demonstrate that, color processing was organized within cortico-subcort‐ ical circuits [11]. It could be presumed that, the neuronal assemblies processing light informa‐ tion as well as their blood flow supply, share analogous topological organization [26]. In men, the distribution of the S-peaks showed that dark elicited the least effect followed by White, Blue, Yellow and Red. This type of summation of responses, with distinction between achro‐ matic and chromatic contrasts along orthogonal axis, on one hand, and between short wavelength and long wavelength on the other, could be presumed to be evidence for stimulus complexity topological organization based on wavelength, in the right hemisphere in men [12, 17, 26]. The latter extends from an area implicated in luminance processing to a much greater area for wavelength-differencing. Similar stimulus complexity topological organization, has been observed in studies involving facial and object stimuli in the right hemisphere in men [26]. In other words, there is a topological organization, based on category-specific stimulus complexity of the functions, located within the ventral temporal cortex of the right hemisphere in men. The latter would be compatible with the findings that thin stripes in V2 contain functional maps where the color of a stimulus is represented by the location of its response activation peak [52, 53]. Conversely, in the left hemisphere in women, at **C**-peaks, frequencydifferencing separated the peak for high frequency color (Blue) from low frequency color (Yellow), but was parallel to the luminance axis. Hence, the differentiation was processmapped only to frequency. Similar process-map model has been proposed for facial processing [26, 34]. It could be suggested that in the left hemisphere in women, there is a category-specific process-mapping system for retrieving color from memory. The latter would be consonant to the proposed distinct map for representation of color in memory [66, 67].

effect varied exponentially with wavelength-differencing activity, while in women, luminance effect varied logarithmically with frequency-differencing activity. (3) The physiologic range of MFV in the MCA territories for the relationship between luminance effect and wavelengthdifferencing activity in men, was narrower than that for frequency-differencing activity in women. The gender-related differences in color processing, which comprised right hemi‐ sphere wavelength-differencing in men, but left hemisphere frequency-differencing in women, suggest that, color processing followed the same rule of lateralization as other major cognitive functions, such as facial processing [26, 27] and general intelligence [30], which implicated the right hemisphere cognitive style in men, and left hemisphere cognitive style in women. Prior studies have suggested that, the right hemisphere was implicated in color processing [20, 21, 22] and the left hemisphere in color memory [61, 62]. Functional magnetic resonance studies demonstrated that memory for colors activated the same cortical regions associated with color perception in the left fusiform gyrus [63, 64]. This has led some investi‐ gators to suggest that, color memory is a constructive process reflecting the synthesis of features each of which are processed in the different cortical regions [65-67]. The functional integration of color processing and luminance effect could provide a model for the study of the functional integration of color processing and color memory within the color space.

158 Advancements and Breakthroughs in Ultrasound Imaging

However, the reason for gender-related cerebral asymmetry for color processing remains to be elucidated. It was suggested that, the neuroadaptation to the physical qualities of light as a wave and as quanta energy, pre-conditioned the right hemisphere for wavelength-differ‐ encing and left hemisphere for frequency-differencing, respectively. A light hypothesis for cerebral asymmetry was postulated, to infer that, the phenotypic neuroadaptation to the environmental physical constraints of light, led to phenotypic evolution and genetic variation of X-Y gene pairs that determined hemispheric asymmetry [12, 17]. The evolutionary trend is towards optimization of perception of the 'whole' environment by functional coupling of the genes for complementarity of both hemispheres within self, and between both genders [12, 17, 26]. The contralateral hemisphere processing of wavelength-differencing and frequencydifferencing was as a result of the inverse relationship between wavelength and frequency. The results further demonstrate that, color processing was organized within cortico-subcort‐ ical circuits [11]. It could be presumed that, the neuronal assemblies processing light informa‐ tion as well as their blood flow supply, share analogous topological organization [26]. In men, the distribution of the S-peaks showed that dark elicited the least effect followed by White, Blue, Yellow and Red. This type of summation of responses, with distinction between achro‐ matic and chromatic contrasts along orthogonal axis, on one hand, and between short wavelength and long wavelength on the other, could be presumed to be evidence for stimulus complexity topological organization based on wavelength, in the right hemisphere in men [12, 17, 26]. The latter extends from an area implicated in luminance processing to a much greater area for wavelength-differencing. Similar stimulus complexity topological organization, has been observed in studies involving facial and object stimuli in the right hemisphere in men [26]. In other words, there is a topological organization, based on category-specific stimulus complexity of the functions, located within the ventral temporal cortex of the right hemisphere in men. The latter would be compatible with the findings that thin stripes in V2 contain functional maps where the color of a stimulus is represented by the location of its response The present work may offer some indication on the neurophysiologic mechanisms underlying vasomotor changes during color processing, which have remained largely unknown until now. The cerebral arteries are innervated by postganglionic nitrergic nerves, originating from the ipsilateral pterygopalatine ganglion, that tonically dilate cerebral arteries in the resting condition [70]. The observed changes in MFV in response to visual stimulations have been related to imbalance between sympathetic vasoconstrictor traffic and vasodilator effects of nitric oxide (NO) [11, 22, 71]. The NO released postsynaptically, diffuses back across the synaptic cleft, to act on the presynaptic terminals, causing increases in presynaptic glutamate release [70, 72], which could account for ipsilateral LTP [11, 72]. Others contend that LTP induced in the visual cortex in animal models is NO dependent [72]. The LTP and LTD recorded non-invasively using *f*TCDS, have shown neuroplasticity during color processing over several hours [11], and hence could be applied to the study of stroke rehabilitation and monitoring of drug effects on N-Methyl- D-aspartate (NMDA) receptors.

Presuming that these mechanisms are present, it could be proposed that in men, wavelengthdifferencing within the right hemisphere by processes of cortical long-term depression (CLTD) and subcortical long-term potentiation (SLTP) [12, 17], occurred simultaneously with con‐ tralateral cortical short-term depression (CSTD) and subcortical short-term potentiation (SSTP) in the left hemisphere, marked by exponential decaying increase in synaptic strength that appears to involve NMDA receptors as well [12, 17, 73, 74], but decays after reaching the asymptotic levels of MFV (Figure 5A-B). Thus, it could be postulated that in men, in the contralateral left hemisphere, memory activation implicated 'exponential expansion' by CSTD and SSTP processes. On the other hand, in women, frequency-differencing within the left hemisphere occurred by processes of cortical long-term potentiation (CLTP) and subcortical long-term depression (SLTD) [11, 12, 17], concurrently, with ipsilateral cortical short-term potentiation (CSTP) and subcortical short-term depression (SSTD) in a selective area of the left hemisphere [11, 12, 17], characterized by logarithmic decaying decrease in synaptic strength that may also involve NMDA receptors, but decays after reaching the asymptotic levels of MFV (Figure 5C). Thus, in women, in the ipsilateral left hemisphere memory activation involved 'logarithmic compression' by CSTP and SSTD processes. Analogous synaptic and cellular activities have been observed in animal experiments [73, 74].

The clinical relevance of the observed profound gender-related differences in cortico-subcort‐ ical activation during color stimulation is yet to be fully realized. The plausible implications of the result that, in men, in the right hemisphere, SLTP and CLTD occurred with wavelengthdifferencing, and conversely, in women in the left hemisphere, CLTP and SLTD occurred with frequency-differencing, is not known. It could be suggested that, there could be a genderrelated difference in the effects of brain lesions in men and women. While in men, subcortical lesions of the right hemisphere may be associated with severe color deficits because of impaired SLTP processes for wavelength-differencing, in women, cortical lesions of the left hemisphere could result in more severe deficits due to inability to form CLTP processes for frequency-differencing. The onset of memory deficits may be characterized by loss of the capability to perform 'exponential expansion' in the memory area of the left hemisphere in men, or 'logarithmic compression' in the selective memory area of the left hemisphere in women. The latter may open the possibility of use of *f*TCD and *f*TCDS to determine early onset of memory deficits in patients with neurodegenerative disorders. Another practical clinical implication of the findings is that, in the structuring of tests of color vision, one eye has to be tested at a time, since binocular interaction may inhibit the responses from color processing neurons due to multiplexing responses in ocular dominance hypercolumns, rather than responses evoked from ocular dominance columns (blobs) [15].

described as the process-map model [34]. Therefore, the proposed models are not mutually exclusive, and this underscores the fact that facial processing does not impose any new constraints on the brain other than those used for other stimuli. It may be suggested that each stimulus was mapped by category into face or non-face, and by process into holistic or analytic. Therefore, a unified category-specific process-mapping system was implemented for either

A New Functional Transcranial Doppler Spectroscopy (fTCDS) Study of Cerebral Asymmetry During..

http://dx.doi.org/10.5772/54877

161

Furthermore, in women, the neuronal assemblies may not have the same orderly topological arrangement as in men; rather the neurons involved in processing cone and rod vision were segregated within the right hemisphere cortical region. Hence, in women the right hemisphere responded to luminance effect and object perception, but showed no category-specific face effect. The latter arrangement explains the observed right lateralization for non-face Paradigm 1, but left lateralization for facial Paradigms 2 and 3. In other words, similar to men, women showed a tendency for holistic mechanism for processing object stimulus in the right hemi‐ sphere, but in contrast to men, they preferred the analytic mechanism for facial perception in the left hemisphere. Therefore, one major observed gender-related difference was that, while men employed a category-specific process-mapping system for facial processing in the right hemisphere, women used a category-specific process-mapping system for facial processing in the left hemisphere. In conclusion, it could be said that men and women use different hemi‐ spheres with complimentary mechanisms to perceive the essence of facial expressions that we

International Institutes of Advanced Research and Training, Chidicon Medical Center,

[1] Albers J: Interaction of color. New Haven: Yale University Press; 1963, 20-21.

[2] Daw N: Goldfish retina: organization for simultaneous color contrast. Science 1968,

[4] Livingstone MS, Hubel DH: Anatomy and physiology of color system in the primate

[3] Land EH, McCann JJ: Lightness and retinex theory. J Opto Soc Am 1971, 61:1-11.

right or left cognitive styles.

come across in our daily life.

Address all correspondence to: info@chidicon.com

visual cortex. J Neurosci 1984, 4:309-356.

**Author details**

Philip C. Njemanze

**References**

Owerri, Imo State, Nigeria

158:942-944.

In conclusion, gender differences in color processing implicated right hemisphere wavelengthdifferencing in men, but left hemisphere frequency-differencing in women. Future research using *f*TCDS technique should explore clinical applications of color processing in stroke rehabilitation, and monitoring of drug effects. Genetic and comparative animal experiments, as well as brain lesion studies are needed to further elucidate mechanisms of gender differences in color processing.

#### **4.2. Gender-related asymmetry during facial processing**

Facial perception occurred in the cortical region of the right hemisphere in men, but in the left in women. Similar observations have been made using transcranial Doppler [27], and electro‐ physiological techniques [28]. Similar gender-related hemisphere differences have been observed at the amygdale for emotionally related stimuli [29], and for performance-related processing [30-32]. Men showed a right lateralization during object processing, but women showed a right tendency or bilateral activation. The observed category-specific face effect was consistent with the concept of category-specific model, which posits a neural module for face category as distinct from non-face [59, 75, 76, 77]. However, others have advocated the existence of alternative models [33, 34].

The *f*TCD technique presumes that, the neuronal assemblies processing light information share analogous topological organization as their blood flow supply, then dark would elicit the least effect, followed by Paradigm 1, Paradigm 2, and Paradigm 3. This type of summation of responses related to stimulus complexity could be presumed as evidence for topological organization of these cortical areas in men. It has been posited that the latter extends from the area implicated in object perception to a much greater area involved in facial perception [26]. This agrees with the object form topology hypothesis proposed by Ishai and colleagues [33]. However, the relatedness of object and facial perception was process-based, and appears to be associated with their common holistic processing strategy in the right hemisphere. Moreover, when the same men were presented with facial Paradigm 3 requiring analytic processing, the left hemisphere was activated. This agrees in principle with the suggestion made by Gauthier that the extrastriate cortex contains areas that are best suited for different computations, and described as the process-map model [34]. Therefore, the proposed models are not mutually exclusive, and this underscores the fact that facial processing does not impose any new constraints on the brain other than those used for other stimuli. It may be suggested that each stimulus was mapped by category into face or non-face, and by process into holistic or analytic. Therefore, a unified category-specific process-mapping system was implemented for either right or left cognitive styles.

Furthermore, in women, the neuronal assemblies may not have the same orderly topological arrangement as in men; rather the neurons involved in processing cone and rod vision were segregated within the right hemisphere cortical region. Hence, in women the right hemisphere responded to luminance effect and object perception, but showed no category-specific face effect. The latter arrangement explains the observed right lateralization for non-face Paradigm 1, but left lateralization for facial Paradigms 2 and 3. In other words, similar to men, women showed a tendency for holistic mechanism for processing object stimulus in the right hemi‐ sphere, but in contrast to men, they preferred the analytic mechanism for facial perception in the left hemisphere. Therefore, one major observed gender-related difference was that, while men employed a category-specific process-mapping system for facial processing in the right hemisphere, women used a category-specific process-mapping system for facial processing in the left hemisphere. In conclusion, it could be said that men and women use different hemi‐ spheres with complimentary mechanisms to perceive the essence of facial expressions that we come across in our daily life.
