We study the suitability of estimating the information conveyed by the responses of the populations of neurons in the brain by using the signal provided by imaging techniques like functional Magnetic Resonance Imaging (fMRI). The fMRI signal is likely to reflect a spatial averaging of the neuronal population activity. On the other hand, knowledge of the activity of each single neuron is needed in order to calculate the information. We explore this potential limitation by means of a simple computational model based on known tuning properties of individual neurons. We investigate the relationship between the information transmitted by the population, the signal change and the signal information as a function of the neuronal parameters. We find that the relationship is in general very different from linear. This result should be taken into account when comparing the information encoded by different brain areas with functional imaging techniques.