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Semantic processing in the left inferior prefrontal cortex: a combined functional magnetic resonance imaging and transcranial magnetic stimulation study.
The involvement of the left inferior prefrontal cortex (LIPC) in phonological processing is well established from both lesion-deficit studies with neurological patients and functional neuroimaging studies of normals. Its involvement in semantic processing, on the other hand, is less clear. Although many imaging studies have demonstrated LIPC activation during semantic tasks, this may be due to implicit phonological processing. This article presents two experiments investigating semantic functions in the LIPC. Results from a functional magnetic resonance imaging experiment demonstrated that both semantic and phonological processing activated a common set of areas within this region. In addition, there was a reliable increase in activation for semantic relative to phonological decisions in the anterior LIPC while the opposite comparison (phonological vs. semantic decisions) revealed an area of enhanced activation within the posterior LIPC. A second experiment used transcranial magnetic stimulation (TMS) to temporarily interfere with neural information processing in the anterior portion of the LIPC to determine whether this region was essential for normal semantic performance. Both repetitive and single pulse TMS significantly slowed subjects' reactions for the semantic but not for the perceptual control task. Our results clarify the functional anatomy of the LIPC by demonstrating that anterior and posterior regions contribute to both semantic and phonological processing, albeit to different extents. In addition, the findings go beyond simply establishing a correlation between semantic processing and activation in the LIPC and demonstrate that a transient disruption of processing selectively interfered with semantic processing.
Opposing alterations in anxiety and species-typical behaviours in serotonin transporter overexpressor and knockout mice.
Human gene association studies have produced conflicting findings regarding the relationship between the 5-HT transporter (5-HTT) and anxiety. In the present study genetically modified mice were utilised to examine the effects of changes in 5-HTT expression on anxiety. In addition, the influence of 5-HTT expression on two innate "species-typical" behaviours (burrowing and marble burying) and body weight was explored. Across a range of models, 5-HTT overexpressing mice displayed reduced anxiety-like behaviour whilst 5-HTT knockout mice showed increased anxiety-like behaviour, compared to wildtype controls. In tests of species-typical behaviour 5-HTT overexpressing mice showed some facilitation whilst 5-HTT knockout mice were impaired. Reciprocal effects were also seen on body weight, as 5-HTT overexpressors were lighter and 5-HTT knockouts were heavier than wildtype controls. These findings show that variation in 5-HTT gene expression produces robust changes in anxiety and species-typical behaviour. Furthermore, the data add further support to findings that variation of 5-HTT expression in the human population is linked to changes in anxiety-related personality traits.
Functional alterations to the nigrostriatal system in mice lacking all three members of the synuclein family.
The synucleins (α, β, and γ) are highly homologous proteins thought to play a role in regulating neurotransmission and are found abundantly in presynaptic terminals. To overcome functional overlap between synuclein proteins and to understand their role in presynaptic signaling from mesostriatal dopaminergic neurons, we produced mice lacking all three members of the synuclein family. The effect on the mesostriatal system was assessed in adult (4- to 14-month-old) animals using a combination of behavioral, biochemical, histological, and electrochemical techniques. Adult triple-synuclein-null (TKO) mice displayed no overt phenotype and no change in the number of midbrain dopaminergic neurons. TKO mice were hyperactive in novel environments and exhibited elevated evoked release of dopamine in the striatum detected with fast-scan cyclic voltammetry. Elevated dopamine release was specific to the dorsal not ventral striatum and was accompanied by a decrease of dopamine tissue content. We confirmed a normal synaptic ultrastructure and a normal abundance of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein complexes in the dorsal striatum. Treatment of TKO animals with drugs affecting dopamine metabolism revealed normal rate of synthesis, enhanced turnover, and reduced presynaptic striatal dopamine stores. Our data uniquely reveal the importance of the synuclein proteins in regulating neurotransmitter release from specific populations of midbrain dopamine neurons through mechanisms that differ from those reported in other neurons. The finding that the complete loss of synucleins leads to changes in dopamine handling by presynaptic terminals specifically in those regions preferentially vulnerable in Parkinson's disease may ultimately inform on the selectivity of the disease process.
Theta-burst transcranial magnetic stimulation to the prefrontal cortex impairs metacognitive visual awareness.
We used a recently developed protocol of transcranial magnetic stimulation (TMS), theta-burst stimulation, to bilaterally depress activity in the dorsolateral prefrontal cortex as subjects performed a visual discrimination task. We found that TMS impaired subjects' ability to discriminate between correct and incorrect stimulus judgments. Specifically, after TMS subjects reported lower visibility levels for correctly identified stimuli, as if they were less fully aware of the quality of their visual information processing. A signal detection theory analysis confirmed that the results reflect a change in metacognitive sensitivity, not just response bias. The effect was specific to metacognition; TMS did not change stimulus discrimination performance, ruling out alternative explanations such as TMS impairing visual attention. Together these results suggest that activations in the prefrontal cortex in brain imaging experiments on visual awareness are not epiphenomena, but rather may reflect a critical metacognitive process.
Depth and size scaling created by the differential perspective of ground plane surfaces
The horizontal gradient of differential vertical size in the two eyes (differential perspective) provides information about the absolute distance to a frontal surface which has been shown to be effective in both size and depth scaling (Rogers and Bradshaw, 1993, Nature, 361). In the real world, however, extensive frontal surfaces are rare and the more typical ground plane surface instead creates a complex, higher-order gradient of differential perspective. It is straightforward to show that these higher-order gradients provide sufficient information to scale the retinal sizes and horizontal disparities of all objects lying on the ground plane surface as well as providing a potential source of information to calibrate binocular vergence. The present experiments were designed to measure the effectiveness of ground plane differential perspective gradients for size and disparity scaling and for vergence calibration. Large visual field stimuli were created by projecting binocular images onto separate ground plane screens in a modified Wheatstone stereoscope configuration. The test objects were triangular wave corrugations in which observers had to judge the corrugation angle (shape task) and the separation between the peaks (size task). The ground plane gradient of differential perspective was manipulated independently of the vergence demands of the simulated surface. The results show that both size and depth constancy were high (70-80%) when the differential perspective gradient and vergence cues were consistent and appropriate. A substantial independent effect of the differential perspective gradient on size and depth constancy was also revealed when it was set in conflict with the vergence demands of the simulated surface.
Induced effects in motion parallax
Ogle's induced-size effect refers to the percept of slant elicited by a difference in vertical size between the left and right half images of a stereoscopic display. The effect is not readily predicted by the geometry of the situation and has been of considerable interest in the stereoscopic literature. Rogers and Koenderink (Nature, 322: 62-63) demonstrated that modulation of the vertical size of a monocular image during lateral head motion produces the impression of a surface slanted in depth - a motion-parallax analogue of the induced-size effect. We investigated motion parallax analogues of the induced-size and induced-shear effects further and compared them with the corresponding stereoscopic versions. During lateral head motion or with binocular stereopsis, vertical-shear and vertical-size transformations produced 'induced effects' of apparent inclination and slant that are not predicted geometrically. With vertical head motion, horizontal-shear and horizontal-size transformations produced similar analogues of the disparity induced effects. Typically, the induced effects were opposite in direction and slightly smaller than the geometric effects. For both stereopsis and motion parallax, relative slant and inclination were more pronounced when the stimulus contained discontinuities in disparity/velocity gradient than for continuous disparity/flow fields. The results have important implications for the processing of disparity and optic flow fields.
Geometric and induced effects in binocular stereopsis and motion parallax.
This paper examines and contrasts motion-parallax analogues of the induced-size and induced-shear effects with the equivalent induced effects from binocular disparity. During lateral head motion or with binocular stereopsis, vertical-shear and vertical-size transformations produced 'induced effects' of apparent inclination and slant that are not predicted geometrically. With vertical head motion, horizontal-shear and horizontal-size transformations produced similar analogues of the disparity induced effects. Typically, the induced effects were opposite in direction and slightly smaller in size than the geometric effects. Local induced-shear and induced-size effects could be elicited from motion parallax, but not from disparity, and were most pronounced when the stimulus contained discontinuities in velocity gradient. The implications of these results are discussed in the context of models of depth perception from disparity and structure from motion.
The stereoscopic anisotropy: individual differences and underlying mechanisms.
Observers are more sensitive to variations in the depth of stereoscopic surfaces in a vertical than in a horizontal direction; however, there are large individual differences in this anisotropy. The authors measured discrimination thresholds for surfaces slanted about a vertical axis or inclined about a horizontal axis for 50 observers. Orientation and spatial frequency discrimination thresholds were also measured. For most observers, thresholds were lower for inclination than for slant and lower for orientation than for spatial frequency. There was a positive correlation between the 2 anisotropies, resulting from positive correlations between (a) orientation and inclination thresholds and (b) spatial frequency and slant thresholds. These results support the notion that surface inclination and slant perception is in part limited by the sensitivity of orientation and spatial frequency mechanisms.
Perceiving in Depth, Volume 2
"The proposed three volumes are the latest installment in Ian Howard's amazing ongoing project of providing the most comprehensive review available anywhereof all aspects of how humans and animals perceive and navigate the three-dimensional ...