Summation

About: Summation is a research topic. Over the lifetime, 954 publications have been published within this topic receiving 45593 citations. The topic is also known as: summation & sum of a sequence.

Brain activity related to temporal summation of C-fiber evoked pain.

Abstract: Temporal summation of "second pain" (TSSP) is considered to be the result of C-fiber-evoked responses of dorsal horn neurons, termed 'windup'. This phenomenon is dependent on stimulus frequency (0.33 Hz) and relevant for central sensitization and chronic pain. Previous brain imaging studies have only been used to characterize neural correlates of second pain but not its temporal summation. We utilized functional magnetic resonance imaging (fMRI) in healthy volunteers to measure brain responses associated with TSSP. Region of interest analysis was used to assess TSSP related brain activation. Eleven pain-free normal subjects underwent fMRI scanning during repetitive heat pulses to the right foot at 0.33 and 0.17 Hz. Stimulus intensities were adjusted to each individual's heat sensitivity to achieve comparable TSSP ratings of moderate pain in all subjects. As predicted, experimental pain ratings showed robust TSSP during 0.33 Hz but not 0.17 Hz stimuli. fMRI statistical maps identified several brain regions with stimulus and frequency dependent activation consistent with TSSP, including contralateral thalamus (THAL), S1, bilateral S2, anterior and posterior insula (INS), mid-anterior cingulate cortex (ACC), and supplemental motor areas (SMA). TSSP ratings were significantly correlated with brain activation in somatosensory areas (THAL, S1, left S2), anterior INS, and ACC. These results show that neural responses related to TSSP are evoked in somatosensory processing areas (THAL, S2), as well as in multiple areas that serve other functions related to pain, such as cognition (ACC, PFC), affect (INS, ACC, PAG), pre-motor activity (SMA, cerebellum), and pain modulation (rostral ACC).

Spatial summation of inhibitory influences in the eye of Limulus, and the mutual interaction of receptor units.

Abstract: The inhibitory influences exerted mutually among the receptor units (ommatidia) of the lateral eye of Limulus are additive. If two groups of receptors are illuminated together the total inhibition they exert on a "test receptor" near them (decrease in the frequency of its nerve impulse discharge in response to light) depends on the combined inhibitory influences exerted by the two groups. If the two groups are widely separated in the eye, their total inhibitory effect on the test receptor equals the sum of the inhibitory effects they each produce separately. If they are close enough together to interact, their effect when acting together is usually less than the sum of their separate effects, since each group inhibits the activity of the other and hence reduces its inhibitory influence. However, the test receptor, or a small group illuminated with it, may interact with the two groups and affect the net inhibitory action. A variety of quantitative effects have been observed for different configurations of three such groups of receptors. The activity of a population of n interacting elements is described by a set of n simultaneous equations, linear in the frequencies of the receptor elements involved. Applied to three interacting receptors or receptor groups equations are derived that account quantitatively for the variety of effects observed in the various experimental configurations of retinal illumination used.

Sensory testing of pathophysiological mechanisms of pain in patients with reflex sympathetic dystrophy.

Abstract: The incidence of 3 sensory abnormalities was studied among 17 patients with a diagnosis of reflex sympathetic dystrophy (RSD) and 14 patients with persistent limb pain following trauma; the extent to which the 3 abnormalities were associated with each other and with the intensity of spontaneous clinical pain were also studied. These abnormalities included (1) heat-induced hyperalgesia (54.8% of 31 patients tested); (2) low-threshold A beta-mediated (45.2%) or high-threshold (54.8%) mechanical allodynia; and (3) slow temporal summation of mechanical allodynia (10 of 29 patients tested). These 3 sensory abnormalities occurred to widely varying extents and were not reliably associated with each other. As hypothesized, patients with temporal summation had significantly more intense spontaneous pain than those who did not demonstrate this sensory characteristic. In contrast, the presence or absence of thermal hyperalgesia and type of allodynia did not appear to influence the intensity of spontaneous pain. These results indicate that variable types of primary afferents (i.e., A beta versus A delta, C) and/or varying extents of abnormal spatial summation mechanisms trigger pain responses among RSD patients and that at least one of these, slow temporal summation, is likely to contribute to the intensity of a patient's ongoing pain.

Spatial summation in the receptive fields of MT neurons.

Abstract: Receptive fields (RFs) of cells in the middle temporal area (MT or V5) of monkeys will often encompass multiple objects under normal image viewing. We therefore have studied how multiple moving stimuli interact when presented within and near the RF of single MT cells. We used moving Gabor function stimuli, <1 degrees in spatial extent and approximately 100 msec in duration, presented on a grid of possible locations over the RF of the cell. Responses to these stimuli were typically robust, and their small spatial and temporal extent allowed detailed mapping of RFs and of interactions between stimuli. The responses to pairs of such stimuli were compared against the responses to the same stimuli presented singly. The responses were substantially less than the sum of the responses to the component stimuli and were well described by a power-law summation model with divisive inhibition. Such divisive inhibition is a key component of recently proposed "normalization" models of cortical physiology and is presumed to arise from lateral interconnections within a region. One open question is whether the normalization occurs only once in primary visual cortex or multiple times in different cortical areas. We addressed this question by exploring the spatial extent over which one stimulus would divide the response to another and found effective normalization from stimuli quite far removed from the RF center. This supports models under which normalization occurs both in MT and in earlier stages.