A medical device for detecting signals, comprising: a first optical assembly comprising a first light emitter and a first light detector; a second optical assembly comprising a second light emitter and a second light detector; wherein said first and second optical assemblies are spatially arranged relative to each other such that the first light detector is arranged to detect emitted light scattered by a volume of tissue delivered from said second light emitter along a first pathway to generate corresponding first detected light intensity output signals, and the second light detector is arranged to detect emitted light scattered by the volume of tissue delivered from said first light emitter along a second pathway different from the first pathway to generate corresponding second detected light intensity output signals; and a processor configured to determine whether a difference between the emitted light detected along the first pathway and the emitted light detected along the second pathway is greater than a predetermined threshold, and to alter sensing by the device in response to the determining whether a difference is greater than the predetermined threshold; characterised in that said medical device further comprising a monitoring module coupled to the processor and configured to determine an attenuation measurement for each wavelength of a plurality of wavelengths of the first pathway and the second pathway in response to the first and second detected light intensity output signals, determine a second derivative of the attenuation measurement for a first wavelength and a second wavelength of a plurality of wavelengths corresponding to the first pathway in response to only the first detected light intensity output signals, determine a second derivative of the attenuation measurement for a first wavelength and a second wavelength of a plurality of wavelengths corresponding to the second pathway in response to only the second detected light intensity output signals, determine a first oxygen saturation measurement corresponding to detected light emitted along the first pathway in response to the determined second derivative associated with the first pathway, and determine a second oxygen saturation measurement corresponding to detected light emitted along the second pathway in response to the determined second derivative associated with the second pathway, and wherein the processor is further configured to compare the first and second oxygen saturation measurement. a first optical assembly comprising a first light emitter and a first light detector; a second optical assembly comprising a second light emitter and a second light detector; wherein said first and second optical assemblies are spatially arranged relative to each other such that the first light detector is arranged to detect emitted light scattered by a volume of tissue delivered from said second light emitter along a first pathway to generate corresponding first detected light intensity output signals, and the second light detector is arranged to detect emitted light scattered by the volume of tissue delivered from said first light emitter along a second pathway different from the first pathway to generate corresponding second detected light intensity output signals; and a processor configured to determine whether a difference between the emitted light detected along the first pathway and the emitted light detected along the second pathway is greater than a predetermined threshold, and to alter sensing by the device in response to the determining whether a difference is greater than the predetermined threshold; characterised in that said medical device further comprising a monitoring module coupled to the processor and configured to determine an attenuation measurement for each wavelength of a plurality of wavelengths of the first pathway and the second pathway in response to the first and second detected light intensity output signals, determine a second derivative of the attenuation measurement for a first wavelength and a second wavelength of a plurality of wavelengths corresponding to the first pathway in response to only the first detected light intensity output signals, determine a second derivative of the attenuation measurement for a first wavelength and a second wavelength of a plurality of wavelengths corresponding to the second pathway in response to only the second detected light intensity output signals, determine a first oxygen saturation measurement corresponding to detected light emitted along the first pathway in response to the determined second derivative associated with the first pathway, and determine a second oxygen saturation measurement corresponding to detected light emitted along the second pathway in response to the determined second derivative associated with the second pathway, and wherein the processor is further configured to compare the first and second oxygen saturation measurement.